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Patch by Anastasi Voitova with with small fixes by me.


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+============================
+"Clang" CFE Internals Manual
+============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document describes some of the more important APIs and internal design
+decisions made in the Clang C front-end.  The purpose of this document is to
+both capture some of this high level information and also describe some of the
+design decisions behind it.  This is meant for people interested in hacking on
+Clang, not for end-users.  The description below is categorized by libraries,
+and does not describe any of the clients of the libraries.
+
+LLVM Support Library
+====================
+
+The LLVM ``libSupport`` library provides many underlying libraries and
+`data-structures <http://llvm.org/docs/ProgrammersManual.html>`_, including
+command line option processing, various containers and a system abstraction
+layer, which is used for file system access.
+
+The Clang "Basic" Library
+=========================
+
+This library certainly needs a better name.  The "basic" library contains a
+number of low-level utilities for tracking and manipulating source buffers,
+locations within the source buffers, diagnostics, tokens, target abstraction,
+and information about the subset of the language being compiled for.
+
+Part of this infrastructure is specific to C (such as the ``TargetInfo``
+class), other parts could be reused for other non-C-based languages
+(``SourceLocation``, ``SourceManager``, ``Diagnostics``, ``FileManager``).
+When and if there is future demand we can figure out if it makes sense to
+introduce a new library, move the general classes somewhere else, or introduce
+some other solution.
+
+We describe the roles of these classes in order of their dependencies.
+
+The Diagnostics Subsystem
+-------------------------
+
+The Clang Diagnostics subsystem is an important part of how the compiler
+communicates with the human.  Diagnostics are the warnings and errors produced
+when the code is incorrect or dubious.  In Clang, each diagnostic produced has
+(at the minimum) a unique ID, an English translation associated with it, a
+:ref:`SourceLocation <SourceLocation>` to "put the caret", and a severity
+(e.g., ``WARNING`` or ``ERROR``).  They can also optionally include a number of
+arguments to the dianostic (which fill in "%0"'s in the string) as well as a
+number of source ranges that related to the diagnostic.
+
+In this section, we'll be giving examples produced by the Clang command line
+driver, but diagnostics can be :ref:`rendered in many different ways
+<DiagnosticClient>` depending on how the ``DiagnosticClient`` interface is
+implemented.  A representative example of a diagnostic is:
+
+.. code-block:: c++
+
+  t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
+  P = (P-42) + Gamma*4;
+      ~~~~~~ ^ ~~~~~~~
+
+In this example, you can see the English translation, the severity (error), you
+can see the source location (the caret ("``^``") and file/line/column info),
+the source ranges "``~~~~``", arguments to the diagnostic ("``int*``" and
+"``_Complex float``").  You'll have to believe me that there is a unique ID
+backing the diagnostic :).
+
+Getting all of this to happen has several steps and involves many moving
+pieces, this section describes them and talks about best practices when adding
+a new diagnostic.
+
+The Diagnostic*Kinds.td files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Diagnostics are created by adding an entry to one of the
+``clang/Basic/Diagnostic*Kinds.td`` files, depending on what library will be
+using it.  From this file, :program:`tblgen` generates the unique ID of the
+diagnostic, the severity of the diagnostic and the English translation + format
+string.
+
+There is little sanity with the naming of the unique ID's right now.  Some
+start with ``err_``, ``warn_``, ``ext_`` to encode the severity into the name.
+Since the enum is referenced in the C++ code that produces the diagnostic, it
+is somewhat useful for it to be reasonably short.
+
+The severity of the diagnostic comes from the set {``NOTE``, ``WARNING``,
+``EXTENSION``, ``EXTWARN``, ``ERROR``}.  The ``ERROR`` severity is used for
+diagnostics indicating the program is never acceptable under any circumstances.
+When an error is emitted, the AST for the input code may not be fully built.
+The ``EXTENSION`` and ``EXTWARN`` severities are used for extensions to the
+language that Clang accepts.  This means that Clang fully understands and can
+represent them in the AST, but we produce diagnostics to tell the user their
+code is non-portable.  The difference is that the former are ignored by
+default, and the later warn by default.  The ``WARNING`` severity is used for
+constructs that are valid in the currently selected source language but that
+are dubious in some way.  The ``NOTE`` level is used to staple more information
+onto previous diagnostics.
+
+These *severities* are mapped into a smaller set (the ``Diagnostic::Level``
+enum, {``Ignored``, ``Note``, ``Warning``, ``Error``, ``Fatal``}) of output
+*levels* by the diagnostics subsystem based on various configuration options.
+Clang internally supports a fully fine grained mapping mechanism that allows
+you to map almost any diagnostic to the output level that you want.  The only
+diagnostics that cannot be mapped are ``NOTE``\ s, which always follow the
+severity of the previously emitted diagnostic and ``ERROR``\ s, which can only
+be mapped to ``Fatal`` (it is not possible to turn an error into a warning, for
+example).
+
+Diagnostic mappings are used in many ways.  For example, if the user specifies
+``-pedantic``, ``EXTENSION`` maps to ``Warning``, if they specify
+``-pedantic-errors``, it turns into ``Error``.  This is used to implement
+options like ``-Wunused_macros``, ``-Wundef`` etc.
+
+Mapping to ``Fatal`` should only be used for diagnostics that are considered so
+severe that error recovery won't be able to recover sensibly from them (thus
+spewing a ton of bogus errors).  One example of this class of error are failure
+to ``#include`` a file.
+
+The Format String
+^^^^^^^^^^^^^^^^^
+
+The format string for the diagnostic is very simple, but it has some power.  It
+takes the form of a string in English with markers that indicate where and how
+arguments to the diagnostic are inserted and formatted.  For example, here are
+some simple format strings:
+
+.. code-block:: c++
+
+  "binary integer literals are an extension"
+  "format string contains '\\0' within the string body"
+  "more '%%' conversions than data arguments"
+  "invalid operands to binary expression (%0 and %1)"
+  "overloaded '%0' must be a %select{unary|binary|unary or binary}2 operator"
+       " (has %1 parameter%s1)"
+
+These examples show some important points of format strings.  You can use any
+plain ASCII character in the diagnostic string except "``%``" without a
+problem, but these are C strings, so you have to use and be aware of all the C
+escape sequences (as in the second example).  If you want to produce a "``%``"
+in the output, use the "``%%``" escape sequence, like the third diagnostic.
+Finally, Clang uses the "``%...[digit]``" sequences to specify where and how
+arguments to the diagnostic are formatted.
+
+Arguments to the diagnostic are numbered according to how they are specified by
+the C++ code that :ref:`produces them <internals-producing-diag>`, and are
+referenced by ``%0`` .. ``%9``.  If you have more than 10 arguments to your
+diagnostic, you are doing something wrong :).  Unlike ``printf``, there is no
+requirement that arguments to the diagnostic end up in the output in the same
+order as they are specified, you could have a format string with "``%1 %0``"
+that swaps them, for example.  The text in between the percent and digit are
+formatting instructions.  If there are no instructions, the argument is just
+turned into a string and substituted in.
+
+Here are some "best practices" for writing the English format string:
+
+* Keep the string short.  It should ideally fit in the 80 column limit of the
+  ``DiagnosticKinds.td`` file.  This avoids the diagnostic wrapping when
+  printed, and forces you to think about the important point you are conveying
+  with the diagnostic.
+* Take advantage of location information.  The user will be able to see the
+  line and location of the caret, so you don't need to tell them that the
+  problem is with the 4th argument to the function: just point to it.
+* Do not capitalize the diagnostic string, and do not end it with a period.
+* If you need to quote something in the diagnostic string, use single quotes.
+
+Diagnostics should never take random English strings as arguments: you
+shouldn't use "``you have a problem with %0``" and pass in things like "``your
+argument``" or "``your return value``" as arguments.  Doing this prevents
+:ref:`translating <internals-diag-translation>` the Clang diagnostics to other
+languages (because they'll get random English words in their otherwise
+localized diagnostic).  The exceptions to this are C/C++ language keywords
+(e.g., ``auto``, ``const``, ``mutable``, etc) and C/C++ operators (``/=``).
+Note that things like "pointer" and "reference" are not keywords.  On the other
+hand, you *can* include anything that comes from the user's source code,
+including variable names, types, labels, etc.  The "``select``" format can be
+used to achieve this sort of thing in a localizable way, see below.
+
+Formatting a Diagnostic Argument
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Arguments to diagnostics are fully typed internally, and come from a couple
+different classes: integers, types, names, and random strings.  Depending on
+the class of the argument, it can be optionally formatted in different ways.
+This gives the ``DiagnosticClient`` information about what the argument means
+without requiring it to use a specific presentation (consider this MVC for
+Clang :).
+
+Here are the different diagnostic argument formats currently supported by
+Clang:
+
+**"s" format**
+
+Example:
+  ``"requires %1 parameter%s1"``
+Class:
+  Integers
+Description:
+  This is a simple formatter for integers that is useful when producing English
+  diagnostics.  When the integer is 1, it prints as nothing.  When the integer
+  is not 1, it prints as "``s``".  This allows some simple grammatical forms to
+  be to be handled correctly, and eliminates the need to use gross things like
+  ``"requires %1 parameter(s)"``.
+
+**"select" format**
+
+Example:
+  ``"must be a %select{unary|binary|unary or binary}2 operator"``
+Class:
+  Integers
+Description:
+  This format specifier is used to merge multiple related diagnostics together
+  into one common one, without requiring the difference to be specified as an
+  English string argument.  Instead of specifying the string, the diagnostic
+  gets an integer argument and the format string selects the numbered option.
+  In this case, the "``%2``" value must be an integer in the range [0..2].  If
+  it is 0, it prints "unary", if it is 1 it prints "binary" if it is 2, it
+  prints "unary or binary".  This allows other language translations to
+  substitute reasonable words (or entire phrases) based on the semantics of the
+  diagnostic instead of having to do things textually.  The selected string
+  does undergo formatting.
+
+**"plural" format**
+
+Example:
+  ``"you have %1 %plural{1:mouse|:mice}1 connected to your computer"``
+Class:
+  Integers
+Description:
+  This is a formatter for complex plural forms.  It is designed to handle even
+  the requirements of languages with very complex plural forms, as many Baltic
+  languages have.  The argument consists of a series of expression/form pairs,
+  separated by ":", where the first form whose expression evaluates to true is
+  the result of the modifier.
+
+  An expression can be empty, in which case it is always true.  See the example
+  at the top.  Otherwise, it is a series of one or more numeric conditions,
+  separated by ",".  If any condition matches, the expression matches.  Each
+  numeric condition can take one of three forms.
+
+  * number: A simple decimal number matches if the argument is the same as the
+    number.  Example: ``"%plural{1:mouse|:mice}4"``
+  * range: A range in square brackets matches if the argument is within the
+    range.  Then range is inclusive on both ends.  Example:
+    ``"%plural{0:none|1:one|[2,5]:some|:many}2"``
+  * modulo: A modulo operator is followed by a number, and equals sign and
+    either a number or a range.  The tests are the same as for plain numbers
+    and ranges, but the argument is taken modulo the number first.  Example:
+    ``"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything else}1"``
+
+  The parser is very unforgiving.  A syntax error, even whitespace, will abort,
+  as will a failure to match the argument against any expression.
+
+**"ordinal" format**
+
+Example:
+  ``"ambiguity in %ordinal0 argument"``
+Class:
+  Integers
+Description:
+  This is a formatter which represents the argument number as an ordinal: the
+  value ``1`` becomes ``1st``, ``3`` becomes ``3rd``, and so on.  Values less
+  than ``1`` are not supported.  This formatter is currently hard-coded to use
+  English ordinals.
+
+**"objcclass" format**
+
+Example:
+  ``"method %objcclass0 not found"``
+Class:
+  ``DeclarationName``
+Description:
+  This is a simple formatter that indicates the ``DeclarationName`` corresponds
+  to an Objective-C class method selector.  As such, it prints the selector
+  with a leading "``+``".
+
+**"objcinstance" format**
+
+Example:
+  ``"method %objcinstance0 not found"``
+Class:
+  ``DeclarationName``
+Description:
+  This is a simple formatter that indicates the ``DeclarationName`` corresponds
+  to an Objective-C instance method selector.  As such, it prints the selector
+  with a leading "``-``".
+
+**"q" format**
+
+Example:
+  ``"candidate found by name lookup is %q0"``
+Class:
+  ``NamedDecl *``
+Description:
+  This formatter indicates that the fully-qualified name of the declaration
+  should be printed, e.g., "``std::vector``" rather than "``vector``".
+
+**"diff" format**
+
+Example:
+  ``"no known conversion %diff{from $ to $|from argument type to parameter type}1,2"``
+Class:
+  ``QualType``
+Description:
+  This formatter takes two ``QualType``\ s and attempts to print a template
+  difference between the two.  If tree printing is off, the text inside the
+  braces before the pipe is printed, with the formatted text replacing the $.
+  If tree printing is on, the text after the pipe is printed and a type tree is
+  printed after the diagnostic message.
+
+It is really easy to add format specifiers to the Clang diagnostics system, but
+they should be discussed before they are added.  If you are creating a lot of
+repetitive diagnostics and/or have an idea for a useful formatter, please bring
+it up on the cfe-dev mailing list.
+
+.. _internals-producing-diag:
+
+Producing the Diagnostic
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Now that you've created the diagnostic in the ``Diagnostic*Kinds.td`` file, you
+need to write the code that detects the condition in question and emits the new
+diagnostic.  Various components of Clang (e.g., the preprocessor, ``Sema``,
+etc.) provide a helper function named "``Diag``".  It creates a diagnostic and
+accepts the arguments, ranges, and other information that goes along with it.
+
+For example, the binary expression error comes from code like this:
+
+.. code-block:: c++
+
+  if (various things that are bad)
+    Diag(Loc, diag::err_typecheck_invalid_operands)
+      << lex->getType() << rex->getType()
+      << lex->getSourceRange() << rex->getSourceRange();
+
+This shows that use of the ``Diag`` method: it takes a location (a
+:ref:`SourceLocation <SourceLocation>` object) and a diagnostic enum value
+(which matches the name from ``Diagnostic*Kinds.td``).  If the diagnostic takes
+arguments, they are specified with the ``<<`` operator: the first argument
+becomes ``%0``, the second becomes ``%1``, etc.  The diagnostic interface
+allows you to specify arguments of many different types, including ``int`` and
+``unsigned`` for integer arguments, ``const char*`` and ``std::string`` for
+string arguments, ``DeclarationName`` and ``const IdentifierInfo *`` for names,
+``QualType`` for types, etc.  ``SourceRange``\ s are also specified with the
+``<<`` operator, but do not have a specific ordering requirement.
+
+As you can see, adding and producing a diagnostic is pretty straightforward.
+The hard part is deciding exactly what you need to say to help the user,
+picking a suitable wording, and providing the information needed to format it
+correctly.  The good news is that the call site that issues a diagnostic should
+be completely independent of how the diagnostic is formatted and in what
+language it is rendered.
+
+Fix-It Hints
+^^^^^^^^^^^^
+
+In some cases, the front end emits diagnostics when it is clear that some small
+change to the source code would fix the problem.  For example, a missing
+semicolon at the end of a statement or a use of deprecated syntax that is
+easily rewritten into a more modern form.  Clang tries very hard to emit the
+diagnostic and recover gracefully in these and other cases.
+
+However, for these cases where the fix is obvious, the diagnostic can be
+annotated with a hint (referred to as a "fix-it hint") that describes how to
+change the code referenced by the diagnostic to fix the problem.  For example,
+it might add the missing semicolon at the end of the statement or rewrite the
+use of a deprecated construct into something more palatable.  Here is one such
+example from the C++ front end, where we warn about the right-shift operator
+changing meaning from C++98 to C++11:
+
+.. code-block:: c++
+
+  test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument
+                         will require parentheses in C++11
+  A<100 >> 2> *a;
+        ^
+    (       )
+
+Here, the fix-it hint is suggesting that parentheses be added, and showing
+exactly where those parentheses would be inserted into the source code.  The
+fix-it hints themselves describe what changes to make to the source code in an
+abstract manner, which the text diagnostic printer renders as a line of
+"insertions" below the caret line.  :ref:`Other diagnostic clients
+<DiagnosticClient>` might choose to render the code differently (e.g., as
+markup inline) or even give the user the ability to automatically fix the
+problem.
+
+Fix-it hints on errors and warnings need to obey these rules:
+
+* Since they are automatically applied if ``-Xclang -fixit`` is passed to the
+  driver, they should only be used when it's very likely they match the user's
+  intent.
+* Clang must recover from errors as if the fix-it had been applied.
+
+If a fix-it can't obey these rules, put the fix-it on a note.  Fix-its on notes
+are not applied automatically.
+
+All fix-it hints are described by the ``FixItHint`` class, instances of which
+should be attached to the diagnostic using the ``<<`` operator in the same way
+that highlighted source ranges and arguments are passed to the diagnostic.
+Fix-it hints can be created with one of three constructors:
+
+* ``FixItHint::CreateInsertion(Loc, Code)``
+
+    Specifies that the given ``Code`` (a string) should be inserted before the
+    source location ``Loc``.
+
+* ``FixItHint::CreateRemoval(Range)``
+
+    Specifies that the code in the given source ``Range`` should be removed.
+
+* ``FixItHint::CreateReplacement(Range, Code)``
+
+    Specifies that the code in the given source ``Range`` should be removed,
+    and replaced with the given ``Code`` string.
+
+.. _DiagnosticClient:
+
+The ``DiagnosticClient`` Interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Once code generates a diagnostic with all of the arguments and the rest of the
+relevant information, Clang needs to know what to do with it.  As previously
+mentioned, the diagnostic machinery goes through some filtering to map a
+severity onto a diagnostic level, then (assuming the diagnostic is not mapped
+to "``Ignore``") it invokes an object that implements the ``DiagnosticClient``
+interface with the information.
+
+It is possible to implement this interface in many different ways.  For
+example, the normal Clang ``DiagnosticClient`` (named
+``TextDiagnosticPrinter``) turns the arguments into strings (according to the
+various formatting rules), prints out the file/line/column information and the
+string, then prints out the line of code, the source ranges, and the caret.
+However, this behavior isn't required.
+
+Another implementation of the ``DiagnosticClient`` interface is the
+``TextDiagnosticBuffer`` class, which is used when Clang is in ``-verify``
+mode.  Instead of formatting and printing out the diagnostics, this
+implementation just captures and remembers the diagnostics as they fly by.
+Then ``-verify`` compares the list of produced diagnostics to the list of
+expected ones.  If they disagree, it prints out its own output.  Full
+documentation for the ``-verify`` mode can be found in the Clang API
+documentation for `VerifyDiagnosticConsumer
+</doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details>`_.
+
+There are many other possible implementations of this interface, and this is
+why we prefer diagnostics to pass down rich structured information in
+arguments.  For example, an HTML output might want declaration names be
+linkified to where they come from in the source.  Another example is that a GUI
+might let you click on typedefs to expand them.  This application would want to
+pass significantly more information about types through to the GUI than a
+simple flat string.  The interface allows this to happen.
+
+.. _internals-diag-translation:
+
+Adding Translations to Clang
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Not possible yet! Diagnostic strings should be written in UTF-8, the client can
+translate to the relevant code page if needed.  Each translation completely
+replaces the format string for the diagnostic.
+
+.. _SourceLocation:
+.. _SourceManager:
+
+The ``SourceLocation`` and ``SourceManager`` classes
+----------------------------------------------------
+
+Strangely enough, the ``SourceLocation`` class represents a location within the
+source code of the program.  Important design points include:
+
+#. ``sizeof(SourceLocation)`` must be extremely small, as these are embedded
+   into many AST nodes and are passed around often.  Currently it is 32 bits.
+#. ``SourceLocation`` must be a simple value object that can be efficiently
+   copied.
+#. We should be able to represent a source location for any byte of any input
+   file.  This includes in the middle of tokens, in whitespace, in trigraphs,
+   etc.
+#. A ``SourceLocation`` must encode the current ``#include`` stack that was
+   active when the location was processed.  For example, if the location
+   corresponds to a token, it should contain the set of ``#include``\ s active
+   when the token was lexed.  This allows us to print the ``#include`` stack
+   for a diagnostic.
+#. ``SourceLocation`` must be able to describe macro expansions, capturing both
+   the ultimate instantiation point and the source of the original character
+   data.
+
+In practice, the ``SourceLocation`` works together with the ``SourceManager``
+class to encode two pieces of information about a location: its spelling
+location and its instantiation location.  For most tokens, these will be the
+same.  However, for a macro expansion (or tokens that came from a ``_Pragma``
+directive) these will describe the location of the characters corresponding to
+the token and the location where the token was used (i.e., the macro
+instantiation point or the location of the ``_Pragma`` itself).
+
+The Clang front-end inherently depends on the location of a token being tracked
+correctly.  If it is ever incorrect, the front-end may get confused and die.
+The reason for this is that the notion of the "spelling" of a ``Token`` in
+Clang depends on being able to find the original input characters for the
+token.  This concept maps directly to the "spelling location" for the token.
+
+``SourceRange`` and ``CharSourceRange``
+---------------------------------------
+
+.. mostly taken from http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html
+
+Clang represents most source ranges by [first, last], where "first" and "last"
+each point to the beginning of their respective tokens.  For example consider
+the ``SourceRange`` of the following statement:
+
+.. code-block:: c++
+
+  x = foo + bar;
+  ^first    ^last
+
+To map from this representation to a character-based representation, the "last"
+location needs to be adjusted to point to (or past) the end of that token with
+either ``Lexer::MeasureTokenLength()`` or ``Lexer::getLocForEndOfToken()``.  For
+the rare cases where character-level source ranges information is needed we use
+the ``CharSourceRange`` class.
+
+The Driver Library
+==================
+
+The clang Driver and library are documented `here <DriverInternals.html>`_.
+
+Precompiled Headers
+===================
+
+Clang supports two implementations of precompiled headers.  The default
+implementation, precompiled headers (:doc:`PCH <PCHInternals>`) uses a
+serialized representation of Clang's internal data structures, encoded with the
+`LLVM bitstream format <http://llvm.org/docs/BitCodeFormat.html>`_.
+Pretokenized headers (`PTH <PTHInternals.html>`_), on the other hand, contain a
+serialized representation of the tokens encountered when preprocessing a header
+(and anything that header includes).
+
+The Frontend Library
+====================
+
+The Frontend library contains functionality useful for building tools on top of
+the Clang libraries, for example several methods for outputting diagnostics.
+
+The Lexer and Preprocessor Library
+==================================
+
+The Lexer library contains several tightly-connected classes that are involved
+with the nasty process of lexing and preprocessing C source code.  The main
+interface to this library for outside clients is the large ``Preprocessor``
+class.  It contains the various pieces of state that are required to coherently
+read tokens out of a translation unit.
+
+The core interface to the ``Preprocessor`` object (once it is set up) is the
+``Preprocessor::Lex`` method, which returns the next :ref:`Token <Token>` from
+the preprocessor stream.  There are two types of token providers that the
+preprocessor is capable of reading from: a buffer lexer (provided by the
+:ref:`Lexer <Lexer>` class) and a buffered token stream (provided by the
+:ref:`TokenLexer <TokenLexer>` class).
+
+.. _Token:
+
+The Token class
+---------------
+
+The ``Token`` class is used to represent a single lexed token.  Tokens are
+intended to be used by the lexer/preprocess and parser libraries, but are not
+intended to live beyond them (for example, they should not live in the ASTs).
+
+Tokens most often live on the stack (or some other location that is efficient
+to access) as the parser is running, but occasionally do get buffered up.  For
+example, macro definitions are stored as a series of tokens, and the C++
+front-end periodically needs to buffer tokens up for tentative parsing and
+various pieces of look-ahead.  As such, the size of a ``Token`` matters.  On a
+32-bit system, ``sizeof(Token)`` is currently 16 bytes.
+
+Tokens occur in two forms: :ref:`annotation tokens <AnnotationToken>` and
+normal tokens.  Normal tokens are those returned by the lexer, annotation
+tokens represent semantic information and are produced by the parser, replacing
+normal tokens in the token stream.  Normal tokens contain the following
+information:
+
+* **A SourceLocation** --- This indicates the location of the start of the
+  token.
+
+* **A length** --- This stores the length of the token as stored in the
+  ``SourceBuffer``.  For tokens that include them, this length includes
+  trigraphs and escaped newlines which are ignored by later phases of the
+  compiler.  By pointing into the original source buffer, it is always possible
+  to get the original spelling of a token completely accurately.
+
+* **IdentifierInfo** --- If a token takes the form of an identifier, and if
+  identifier lookup was enabled when the token was lexed (e.g., the lexer was
+  not reading in "raw" mode) this contains a pointer to the unique hash value
+  for the identifier.  Because the lookup happens before keyword
+  identification, this field is set even for language keywords like "``for``".
+
+* **TokenKind** --- This indicates the kind of token as classified by the
+  lexer.  This includes things like ``tok::starequal`` (for the "``*=``"
+  operator), ``tok::ampamp`` for the "``&&``" token, and keyword values (e.g.,
+  ``tok::kw_for``) for identifiers that correspond to keywords.  Note that
+  some tokens can be spelled multiple ways.  For example, C++ supports
+  "operator keywords", where things like "``and``" are treated exactly like the
+  "``&&``" operator.  In these cases, the kind value is set to ``tok::ampamp``,
+  which is good for the parser, which doesn't have to consider both forms.  For
+  something that cares about which form is used (e.g., the preprocessor
+  "stringize" operator) the spelling indicates the original form.
+
+* **Flags** --- There are currently four flags tracked by the
+  lexer/preprocessor system on a per-token basis:
+
+  #. **StartOfLine** --- This was the first token that occurred on its input
+     source line.
+  #. **LeadingSpace** --- There was a space character either immediately before
+     the token or transitively before the token as it was expanded through a
+     macro.  The definition of this flag is very closely defined by the
+     stringizing requirements of the preprocessor.
+  #. **DisableExpand** --- This flag is used internally to the preprocessor to
+     represent identifier tokens which have macro expansion disabled.  This
+     prevents them from being considered as candidates for macro expansion ever
+     in the future.
+  #. **NeedsCleaning** --- This flag is set if the original spelling for the
+     token includes a trigraph or escaped newline.  Since this is uncommon,
+     many pieces of code can fast-path on tokens that did not need cleaning.
+
+One interesting (and somewhat unusual) aspect of normal tokens is that they
+don't contain any semantic information about the lexed value.  For example, if
+the token was a pp-number token, we do not represent the value of the number
+that was lexed (this is left for later pieces of code to decide).
+Additionally, the lexer library has no notion of typedef names vs variable
+names: both are returned as identifiers, and the parser is left to decide
+whether a specific identifier is a typedef or a variable (tracking this
+requires scope information among other things).  The parser can do this
+translation by replacing tokens returned by the preprocessor with "Annotation
+Tokens".
+
+.. _AnnotationToken:
+
+Annotation Tokens
+-----------------
+
+Annotation tokens are tokens that are synthesized by the parser and injected
+into the preprocessor's token stream (replacing existing tokens) to record
+semantic information found by the parser.  For example, if "``foo``" is found
+to be a typedef, the "``foo``" ``tok::identifier`` token is replaced with an
+``tok::annot_typename``.  This is useful for a couple of reasons: 1) this makes
+it easy to handle qualified type names (e.g., "``foo::bar::baz<42>::t``") in
+C++ as a single "token" in the parser.  2) if the parser backtracks, the
+reparse does not need to redo semantic analysis to determine whether a token
+sequence is a variable, type, template, etc.
+
+Annotation tokens are created by the parser and reinjected into the parser's
+token stream (when backtracking is enabled).  Because they can only exist in
+tokens that the preprocessor-proper is done with, it doesn't need to keep
+around flags like "start of line" that the preprocessor uses to do its job.
+Additionally, an annotation token may "cover" a sequence of preprocessor tokens
+(e.g., "``a::b::c``" is five preprocessor tokens).  As such, the valid fields
+of an annotation token are different than the fields for a normal token (but
+they are multiplexed into the normal ``Token`` fields):
+
+* **SourceLocation "Location"** --- The ``SourceLocation`` for the annotation
+  token indicates the first token replaced by the annotation token.  In the
+  example above, it would be the location of the "``a``" identifier.
+* **SourceLocation "AnnotationEndLoc"** --- This holds the location of the last
+  token replaced with the annotation token.  In the example above, it would be
+  the location of the "``c``" identifier.
+* **void* "AnnotationValue"** --- This contains an opaque object that the
+  parser gets from ``Sema``.  The parser merely preserves the information for
+  ``Sema`` to later interpret based on the annotation token kind.
+* **TokenKind "Kind"** --- This indicates the kind of Annotation token this is.
+  See below for the different valid kinds.
+
+Annotation tokens currently come in three kinds:
+
+#. **tok::annot_typename**: This annotation token represents a resolved
+   typename token that is potentially qualified.  The ``AnnotationValue`` field
+   contains the ``QualType`` returned by ``Sema::getTypeName()``, possibly with
+   source location information attached.
+#. **tok::annot_cxxscope**: This annotation token represents a C++ scope
+   specifier, such as "``A::B::``".  This corresponds to the grammar
+   productions "*::*" and "*:: [opt] nested-name-specifier*".  The
+   ``AnnotationValue`` pointer is a ``NestedNameSpecifier *`` returned by the
+   ``Sema::ActOnCXXGlobalScopeSpecifier`` and
+   ``Sema::ActOnCXXNestedNameSpecifier`` callbacks.
+#. **tok::annot_template_id**: This annotation token represents a C++
+   template-id such as "``foo<int, 4>``", where "``foo``" is the name of a
+   template.  The ``AnnotationValue`` pointer is a pointer to a ``malloc``'d
+   ``TemplateIdAnnotation`` object.  Depending on the context, a parsed
+   template-id that names a type might become a typename annotation token (if
+   all we care about is the named type, e.g., because it occurs in a type
+   specifier) or might remain a template-id token (if we want to retain more
+   source location information or produce a new type, e.g., in a declaration of
+   a class template specialization).  template-id annotation tokens that refer
+   to a type can be "upgraded" to typename annotation tokens by the parser.
+
+As mentioned above, annotation tokens are not returned by the preprocessor,
+they are formed on demand by the parser.  This means that the parser has to be
+aware of cases where an annotation could occur and form it where appropriate.
+This is somewhat similar to how the parser handles Translation Phase 6 of C99:
+String Concatenation (see C99 5.1.1.2).  In the case of string concatenation,
+the preprocessor just returns distinct ``tok::string_literal`` and
+``tok::wide_string_literal`` tokens and the parser eats a sequence of them
+wherever the grammar indicates that a string literal can occur.
+
+In order to do this, whenever the parser expects a ``tok::identifier`` or
+``tok::coloncolon``, it should call the ``TryAnnotateTypeOrScopeToken`` or
+``TryAnnotateCXXScopeToken`` methods to form the annotation token.  These
+methods will maximally form the specified annotation tokens and replace the
+current token with them, if applicable.  If the current tokens is not valid for
+an annotation token, it will remain an identifier or "``::``" token.
+
+.. _Lexer:
+
+The ``Lexer`` class
+-------------------
+
+The ``Lexer`` class provides the mechanics of lexing tokens out of a source
+buffer and deciding what they mean.  The ``Lexer`` is complicated by the fact
+that it operates on raw buffers that have not had spelling eliminated (this is
+a necessity to get decent performance), but this is countered with careful
+coding as well as standard performance techniques (for example, the comment
+handling code is vectorized on X86 and PowerPC hosts).
+
+The lexer has a couple of interesting modal features:
+
+* The lexer can operate in "raw" mode.  This mode has several features that
+  make it possible to quickly lex the file (e.g., it stops identifier lookup,
+  doesn't specially handle preprocessor tokens, handles EOF differently, etc).
+  This mode is used for lexing within an "``#if 0``" block, for example.
+* The lexer can capture and return comments as tokens.  This is required to
+  support the ``-C`` preprocessor mode, which passes comments through, and is
+  used by the diagnostic checker to identifier expect-error annotations.
+* The lexer can be in ``ParsingFilename`` mode, which happens when
+  preprocessing after reading a ``#include`` directive.  This mode changes the
+  parsing of "``<``" to return an "angled string" instead of a bunch of tokens
+  for each thing within the filename.
+* When parsing a preprocessor directive (after "``#``") the
+  ``ParsingPreprocessorDirective`` mode is entered.  This changes the parser to
+  return EOD at a newline.
+* The ``Lexer`` uses a ``LangOptions`` object to know whether trigraphs are
+  enabled, whether C++ or ObjC keywords are recognized, etc.
+
+In addition to these modes, the lexer keeps track of a couple of other features
+that are local to a lexed buffer, which change as the buffer is lexed:
+
+* The ``Lexer`` uses ``BufferPtr`` to keep track of the current character being
+  lexed.
+* The ``Lexer`` uses ``IsAtStartOfLine`` to keep track of whether the next
+  lexed token will start with its "start of line" bit set.
+* The ``Lexer`` keeps track of the current "``#if``" directives that are active
+  (which can be nested).
+* The ``Lexer`` keeps track of an :ref:`MultipleIncludeOpt
+  <MultipleIncludeOpt>` object, which is used to detect whether the buffer uses
+  the standard "``#ifndef XX`` / ``#define XX``" idiom to prevent multiple
+  inclusion.  If a buffer does, subsequent includes can be ignored if the
+  "``XX``" macro is defined.
+
+.. _TokenLexer:
+
+The ``TokenLexer`` class
+------------------------
+
+The ``TokenLexer`` class is a token provider that returns tokens from a list of
+tokens that came from somewhere else.  It typically used for two things: 1)
+returning tokens from a macro definition as it is being expanded 2) returning
+tokens from an arbitrary buffer of tokens.  The later use is used by
+``_Pragma`` and will most likely be used to handle unbounded look-ahead for the
+C++ parser.
+
+.. _MultipleIncludeOpt:
+
+The ``MultipleIncludeOpt`` class
+--------------------------------
+
+The ``MultipleIncludeOpt`` class implements a really simple little state
+machine that is used to detect the standard "``#ifndef XX`` / ``#define XX``"
+idiom that people typically use to prevent multiple inclusion of headers.  If a
+buffer uses this idiom and is subsequently ``#include``'d, the preprocessor can
+simply check to see whether the guarding condition is defined or not.  If so,
+the preprocessor can completely ignore the include of the header.
+
+The Parser Library
+==================
+
+The AST Library
+===============
+
+.. _Type:
+
+The ``Type`` class and its subclasses
+-------------------------------------
+
+The ``Type`` class (and its subclasses) are an important part of the AST.
+Types are accessed through the ``ASTContext`` class, which implicitly creates
+and uniques them as they are needed.  Types have a couple of non-obvious
+features: 1) they do not capture type qualifiers like ``const`` or ``volatile``
+(see :ref:`QualType <QualType>`), and 2) they implicitly capture typedef
+information.  Once created, types are immutable (unlike decls).
+
+Typedefs in C make semantic analysis a bit more complex than it would be without
+them.  The issue is that we want to capture typedef information and represent it
+in the AST perfectly, but the semantics of operations need to "see through"
+typedefs.  For example, consider this code:
+
+.. code-block:: c++
+
+  void func() {
+    typedef int foo;
+    foo X, *Y;
+    typedef foo *bar;
+    bar Z;
+    *X; // error
+    **Y; // error
+    **Z; // error
+  }
+
+The code above is illegal, and thus we expect there to be diagnostics emitted
+on the annotated lines.  In this example, we expect to get:
+
+.. code-block:: c++
+
+  test.c:6:1: error: indirection requires pointer operand ('foo' invalid)
+    *X; // error
+    ^~
+  test.c:7:1: error: indirection requires pointer operand ('foo' invalid)
+    **Y; // error
+    ^~~
+  test.c:8:1: error: indirection requires pointer operand ('foo' invalid)
+    **Z; // error
+    ^~~
+
+While this example is somewhat silly, it illustrates the point: we want to
+retain typedef information where possible, so that we can emit errors about
+"``std::string``" instead of "``std::basic_string<char, std:...``".  Doing this
+requires properly keeping typedef information (for example, the type of ``X``
+is "``foo``", not "``int``"), and requires properly propagating it through the
+various operators (for example, the type of ``*Y`` is "``foo``", not
+"``int``").  In order to retain this information, the type of these expressions
+is an instance of the ``TypedefType`` class, which indicates that the type of
+these expressions is a typedef for "``foo``".
+
+Representing types like this is great for diagnostics, because the
+user-specified type is always immediately available.  There are two problems
+with this: first, various semantic checks need to make judgements about the
+*actual structure* of a type, ignoring typedefs.  Second, we need an efficient
+way to query whether two types are structurally identical to each other,
+ignoring typedefs.  The solution to both of these problems is the idea of
+canonical types.
+
+Canonical Types
+^^^^^^^^^^^^^^^
+
+Every instance of the ``Type`` class contains a canonical type pointer.  For
+simple types with no typedefs involved (e.g., "``int``", "``int*``",
+"``int**``"), the type just points to itself.  For types that have a typedef
+somewhere in their structure (e.g., "``foo``", "``foo*``", "``foo**``",
+"``bar``"), the canonical type pointer points to their structurally equivalent
+type without any typedefs (e.g., "``int``", "``int*``", "``int**``", and
+"``int*``" respectively).
+
+This design provides a constant time operation (dereferencing the canonical type
+pointer) that gives us access to the structure of types.  For example, we can
+trivially tell that "``bar``" and "``foo*``" are the same type by dereferencing
+their canonical type pointers and doing a pointer comparison (they both point
+to the single "``int*``" type).
+
+Canonical types and typedef types bring up some complexities that must be
+carefully managed.  Specifically, the ``isa``/``cast``/``dyn_cast`` operators
+generally shouldn't be used in code that is inspecting the AST.  For example,
+when type checking the indirection operator (unary "``*``" on a pointer), the
+type checker must verify that the operand has a pointer type.  It would not be
+correct to check that with "``isa<PointerType>(SubExpr->getType())``", because
+this predicate would fail if the subexpression had a typedef type.
+
+The solution to this problem are a set of helper methods on ``Type``, used to
+check their properties.  In this case, it would be correct to use
+"``SubExpr->getType()->isPointerType()``" to do the check.  This predicate will
+return true if the *canonical type is a pointer*, which is true any time the
+type is structurally a pointer type.  The only hard part here is remembering
+not to use the ``isa``/``cast``/``dyn_cast`` operations.
+
+The second problem we face is how to get access to the pointer type once we
+know it exists.  To continue the example, the result type of the indirection
+operator is the pointee type of the subexpression.  In order to determine the
+type, we need to get the instance of ``PointerType`` that best captures the
+typedef information in the program.  If the type of the expression is literally
+a ``PointerType``, we can return that, otherwise we have to dig through the
+typedefs to find the pointer type.  For example, if the subexpression had type
+"``foo*``", we could return that type as the result.  If the subexpression had
+type "``bar``", we want to return "``foo*``" (note that we do *not* want
+"``int*``").  In order to provide all of this, ``Type`` has a
+``getAsPointerType()`` method that checks whether the type is structurally a
+``PointerType`` and, if so, returns the best one.  If not, it returns a null
+pointer.
+
+This structure is somewhat mystical, but after meditating on it, it will make
+sense to you :).
+
+.. _QualType:
+
+The ``QualType`` class
+----------------------
+
+The ``QualType`` class is designed as a trivial value class that is small,
+passed by-value and is efficient to query.  The idea of ``QualType`` is that it
+stores the type qualifiers (``const``, ``volatile``, ``restrict``, plus some
+extended qualifiers required by language extensions) separately from the types
+themselves.  ``QualType`` is conceptually a pair of "``Type*``" and the bits
+for these type qualifiers.
+
+By storing the type qualifiers as bits in the conceptual pair, it is extremely
+efficient to get the set of qualifiers on a ``QualType`` (just return the field
+of the pair), add a type qualifier (which is a trivial constant-time operation
+that sets a bit), and remove one or more type qualifiers (just return a
+``QualType`` with the bitfield set to empty).
+
+Further, because the bits are stored outside of the type itself, we do not need
+to create duplicates of types with different sets of qualifiers (i.e. there is
+only a single heap allocated "``int``" type: "``const int``" and "``volatile
+const int``" both point to the same heap allocated "``int``" type).  This
+reduces the heap size used to represent bits and also means we do not have to
+consider qualifiers when uniquing types (:ref:`Type <Type>` does not even
+contain qualifiers).
+
+In practice, the two most common type qualifiers (``const`` and ``restrict``)
+are stored in the low bits of the pointer to the ``Type`` object, together with
+a flag indicating whether extended qualifiers are present (which must be
+heap-allocated).  This means that ``QualType`` is exactly the same size as a
+pointer.
+
+.. _DeclarationName:
+
+Declaration names
+-----------------
+
+The ``DeclarationName`` class represents the name of a declaration in Clang.
+Declarations in the C family of languages can take several different forms.
+Most declarations are named by simple identifiers, e.g., "``f``" and "``x``" in
+the function declaration ``f(int x)``.  In C++, declaration names can also name
+class constructors ("``Class``" in ``struct Class { Class(); }``), class
+destructors ("``~Class``"), overloaded operator names ("``operator+``"), and
+conversion functions ("``operator void const *``").  In Objective-C,
+declaration names can refer to the names of Objective-C methods, which involve
+the method name and the parameters, collectively called a *selector*, e.g.,
+"``setWidth:height:``".  Since all of these kinds of entities --- variables,
+functions, Objective-C methods, C++ constructors, destructors, and operators
+--- are represented as subclasses of Clang's common ``NamedDecl`` class,
+``DeclarationName`` is designed to efficiently represent any kind of name.
+
+Given a ``DeclarationName`` ``N``, ``N.getNameKind()`` will produce a value
+that describes what kind of name ``N`` stores.  There are 8 options (all of the
+names are inside the ``DeclarationName`` class).
+
+``Identifier``
+
+  The name is a simple identifier.  Use ``N.getAsIdentifierInfo()`` to retrieve
+  the corresponding ``IdentifierInfo*`` pointing to the actual identifier.
+  Note that C++ overloaded operators (e.g., "``operator+``") are represented as
+  special kinds of identifiers.  Use ``IdentifierInfo``'s
+  ``getOverloadedOperatorID`` function to determine whether an identifier is an
+  overloaded operator name.
+
+``ObjCZeroArgSelector``, ``ObjCOneArgSelector``, ``ObjCMultiArgSelector``
+
+  The name is an Objective-C selector, which can be retrieved as a ``Selector``
+  instance via ``N.getObjCSelector()``.  The three possible name kinds for
+  Objective-C reflect an optimization within the ``DeclarationName`` class:
+  both zero- and one-argument selectors are stored as a masked
+  ``IdentifierInfo`` pointer, and therefore require very little space, since
+  zero- and one-argument selectors are far more common than multi-argument
+  selectors (which use a different structure).
+
+``CXXConstructorName``
+
+  The name is a C++ constructor name.  Use ``N.getCXXNameType()`` to retrieve
+  the :ref:`type <QualType>` that this constructor is meant to construct.  The
+  type is always the canonical type, since all constructors for a given type
+  have the same name.
+
+``CXXDestructorName``
+
+  The name is a C++ destructor name.  Use ``N.getCXXNameType()`` to retrieve
+  the :ref:`type <QualType>` whose destructor is being named.  This type is
+  always a canonical type.
+
+``CXXConversionFunctionName``
+
+  The name is a C++ conversion function.  Conversion functions are named
+  according to the type they convert to, e.g., "``operator void const *``".
+  Use ``N.getCXXNameType()`` to retrieve the type that this conversion function
+  converts to.  This type is always a canonical type.
+
+``CXXOperatorName``
+
+  The name is a C++ overloaded operator name.  Overloaded operators are named
+  according to their spelling, e.g., "``operator+``" or "``operator new []``".
+  Use ``N.getCXXOverloadedOperator()`` to retrieve the overloaded operator (a
+  value of type ``OverloadedOperatorKind``).
+
+``DeclarationName``\ s are cheap to create, copy, and compare.  They require
+only a single pointer's worth of storage in the common cases (identifiers,
+zero- and one-argument Objective-C selectors) and use dense, uniqued storage
+for the other kinds of names.  Two ``DeclarationName``\ s can be compared for
+equality (``==``, ``!=``) using a simple bitwise comparison, can be ordered
+with ``<``, ``>``, ``<=``, and ``>=`` (which provide a lexicographical ordering
+for normal identifiers but an unspecified ordering for other kinds of names),
+and can be placed into LLVM ``DenseMap``\ s and ``DenseSet``\ s.
+
+``DeclarationName`` instances can be created in different ways depending on
+what kind of name the instance will store.  Normal identifiers
+(``IdentifierInfo`` pointers) and Objective-C selectors (``Selector``) can be
+implicitly converted to ``DeclarationNames``.  Names for C++ constructors,
+destructors, conversion functions, and overloaded operators can be retrieved
+from the ``DeclarationNameTable``, an instance of which is available as
+``ASTContext::DeclarationNames``.  The member functions
+``getCXXConstructorName``, ``getCXXDestructorName``,
+``getCXXConversionFunctionName``, and ``getCXXOperatorName``, respectively,
+return ``DeclarationName`` instances for the four kinds of C++ special function
+names.
+
+.. _DeclContext:
+
+Declaration contexts
+--------------------
+
+Every declaration in a program exists within some *declaration context*, such
+as a translation unit, namespace, class, or function.  Declaration contexts in
+Clang are represented by the ``DeclContext`` class, from which the various
+declaration-context AST nodes (``TranslationUnitDecl``, ``NamespaceDecl``,
+``RecordDecl``, ``FunctionDecl``, etc.) will derive.  The ``DeclContext`` class
+provides several facilities common to each declaration context:
+
+Source-centric vs. Semantics-centric View of Declarations
+
+  ``DeclContext`` provides two views of the declarations stored within a
+  declaration context.  The source-centric view accurately represents the
+  program source code as written, including multiple declarations of entities
+  where present (see the section :ref:`Redeclarations and Overloads
+  <Redeclarations>`), while the semantics-centric view represents the program
+  semantics.  The two views are kept synchronized by semantic analysis while
+  the ASTs are being constructed.
+
+Storage of declarations within that context
+
+  Every declaration context can contain some number of declarations.  For
+  example, a C++ class (represented by ``RecordDecl``) contains various member
+  functions, fields, nested types, and so on.  All of these declarations will
+  be stored within the ``DeclContext``, and one can iterate over the
+  declarations via [``DeclContext::decls_begin()``,
+  ``DeclContext::decls_end()``).  This mechanism provides the source-centric
+  view of declarations in the context.
+
+Lookup of declarations within that context
+
+  The ``DeclContext`` structure provides efficient name lookup for names within
+  that declaration context.  For example, if ``N`` is a namespace we can look
+  for the name ``N::f`` using ``DeclContext::lookup``.  The lookup itself is
+  based on a lazily-constructed array (for declaration contexts with a small
+  number of declarations) or hash table (for declaration contexts with more
+  declarations).  The lookup operation provides the semantics-centric view of
+  the declarations in the context.
+
+Ownership of declarations
+
+  The ``DeclContext`` owns all of the declarations that were declared within
+  its declaration context, and is responsible for the management of their
+  memory as well as their (de-)serialization.
+
+All declarations are stored within a declaration context, and one can query
+information about the context in which each declaration lives.  One can
+retrieve the ``DeclContext`` that contains a particular ``Decl`` using
+``Decl::getDeclContext``.  However, see the section
+:ref:`LexicalAndSemanticContexts` for more information about how to interpret
+this context information.
+
+.. _Redeclarations:
+
+Redeclarations and Overloads
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Within a translation unit, it is common for an entity to be declared several
+times.  For example, we might declare a function "``f``" and then later
+re-declare it as part of an inlined definition:
+
+.. code-block:: c++
+
+  void f(int x, int y, int z = 1);
+
+  inline void f(int x, int y, int z) { /* ...  */ }
+
+The representation of "``f``" differs in the source-centric and
+semantics-centric views of a declaration context.  In the source-centric view,
+all redeclarations will be present, in the order they occurred in the source
+code, making this view suitable for clients that wish to see the structure of
+the source code.  In the semantics-centric view, only the most recent "``f``"
+will be found by the lookup, since it effectively replaces the first
+declaration of "``f``".
+
+In the semantics-centric view, overloading of functions is represented
+explicitly.  For example, given two declarations of a function "``g``" that are
+overloaded, e.g.,
+
+.. code-block:: c++
+
+  void g();
+  void g(int);
+
+the ``DeclContext::lookup`` operation will return a
+``DeclContext::lookup_result`` that contains a range of iterators over
+declarations of "``g``".  Clients that perform semantic analysis on a program
+that is not concerned with the actual source code will primarily use this
+semantics-centric view.
+
+.. _LexicalAndSemanticContexts:
+
+Lexical and Semantic Contexts
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Each declaration has two potentially different declaration contexts: a
+*lexical* context, which corresponds to the source-centric view of the
+declaration context, and a *semantic* context, which corresponds to the
+semantics-centric view.  The lexical context is accessible via
+``Decl::getLexicalDeclContext`` while the semantic context is accessible via
+``Decl::getDeclContext``, both of which return ``DeclContext`` pointers.  For
+most declarations, the two contexts are identical.  For example:
+
+.. code-block:: c++
+
+  class X {
+  public:
+    void f(int x);
+  };
+
+Here, the semantic and lexical contexts of ``X::f`` are the ``DeclContext``
+associated with the class ``X`` (itself stored as a ``RecordDecl`` AST node).
+However, we can now define ``X::f`` out-of-line:
+
+.. code-block:: c++
+
+  void X::f(int x = 17) { /* ...  */ }
+
+This definition of "``f``" has different lexical and semantic contexts.  The
+lexical context corresponds to the declaration context in which the actual
+declaration occurred in the source code, e.g., the translation unit containing
+``X``.  Thus, this declaration of ``X::f`` can be found by traversing the
+declarations provided by [``decls_begin()``, ``decls_end()``) in the
+translation unit.
+
+The semantic context of ``X::f`` corresponds to the class ``X``, since this
+member function is (semantically) a member of ``X``.  Lookup of the name ``f``
+into the ``DeclContext`` associated with ``X`` will then return the definition
+of ``X::f`` (including information about the default argument).
+
+Transparent Declaration Contexts
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In C and C++, there are several contexts in which names that are logically
+declared inside another declaration will actually "leak" out into the enclosing
+scope from the perspective of name lookup.  The most obvious instance of this
+behavior is in enumeration types, e.g.,
+
+.. code-block:: c++
+
+  enum Color {
+    Red,
+    Green,
+    Blue
+  };
+
+Here, ``Color`` is an enumeration, which is a declaration context that contains
+the enumerators ``Red``, ``Green``, and ``Blue``.  Thus, traversing the list of
+declarations contained in the enumeration ``Color`` will yield ``Red``,
+``Green``, and ``Blue``.  However, outside of the scope of ``Color`` one can
+name the enumerator ``Red`` without qualifying the name, e.g.,
+
+.. code-block:: c++
+
+  Color c = Red;
+
+There are other entities in C++ that provide similar behavior.  For example,
+linkage specifications that use curly braces:
+
+.. code-block:: c++
+
+  extern "C" {
+    void f(int);
+    void g(int);
+  }
+  // f and g are visible here
+
+For source-level accuracy, we treat the linkage specification and enumeration
+type as a declaration context in which its enclosed declarations ("``Red``",
+"``Green``", and "``Blue``"; "``f``" and "``g``") are declared.  However, these
+declarations are visible outside of the scope of the declaration context.
+
+These language features (and several others, described below) have roughly the
+same set of requirements: declarations are declared within a particular lexical
+context, but the declarations are also found via name lookup in scopes
+enclosing the declaration itself.  This feature is implemented via
+*transparent* declaration contexts (see
+``DeclContext::isTransparentContext()``), whose declarations are visible in the
+nearest enclosing non-transparent declaration context.  This means that the
+lexical context of the declaration (e.g., an enumerator) will be the
+transparent ``DeclContext`` itself, as will the semantic context, but the
+declaration will be visible in every outer context up to and including the
+first non-transparent declaration context (since transparent declaration
+contexts can be nested).
+
+The transparent ``DeclContext``\ s are:
+
+* Enumerations (but not C++11 "scoped enumerations"):
+
+  .. code-block:: c++
+
+    enum Color {
+      Red,
+      Green,
+      Blue
+    };
+    // Red, Green, and Blue are in scope
+
+* C++ linkage specifications:
+
+  .. code-block:: c++
+
+    extern "C" {
+      void f(int);
+      void g(int);
+    }
+    // f and g are in scope
+
+* Anonymous unions and structs:
+
+  .. code-block:: c++
+
+    struct LookupTable {
+      bool IsVector;
+      union {
+        std::vector<Item> *Vector;
+        std::set<Item> *Set;
+      };
+    };
+
+    LookupTable LT;
+    LT.Vector = 0; // Okay: finds Vector inside the unnamed union
+
+* C++11 inline namespaces:
+
+  .. code-block:: c++
+
+    namespace mylib {
+      inline namespace debug {
+        class X;
+      }
+    }
+    mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
+
+.. _MultiDeclContext:
+
+Multiply-Defined Declaration Contexts
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+C++ namespaces have the interesting --- and, so far, unique --- property that
+the namespace can be defined multiple times, and the declarations provided by
+each namespace definition are effectively merged (from the semantic point of
+view).  For example, the following two code snippets are semantically
+indistinguishable:
+
+.. code-block:: c++
+
+  // Snippet #1:
+  namespace N {
+    void f();
+  }
+  namespace N {
+    void f(int);
+  }
+
+  // Snippet #2:
+  namespace N {
+    void f();
+    void f(int);
+  }
+
+In Clang's representation, the source-centric view of declaration contexts will
+actually have two separate ``NamespaceDecl`` nodes in Snippet #1, each of which
+is a declaration context that contains a single declaration of "``f``".
+However, the semantics-centric view provided by name lookup into the namespace
+``N`` for "``f``" will return a ``DeclContext::lookup_result`` that contains a
+range of iterators over declarations of "``f``".
+
+``DeclContext`` manages multiply-defined declaration contexts internally.  The
+function ``DeclContext::getPrimaryContext`` retrieves the "primary" context for
+a given ``DeclContext`` instance, which is the ``DeclContext`` responsible for
+maintaining the lookup table used for the semantics-centric view.  Given the
+primary context, one can follow the chain of ``DeclContext`` nodes that define
+additional declarations via ``DeclContext::getNextContext``.  Note that these
+functions are used internally within the lookup and insertion methods of the
+``DeclContext``, so the vast majority of clients can ignore them.
+
+.. _CFG:
+
+The ``CFG`` class
+-----------------
+
+The ``CFG`` class is designed to represent a source-level control-flow graph
+for a single statement (``Stmt*``).  Typically instances of ``CFG`` are
+constructed for function bodies (usually an instance of ``CompoundStmt``), but
+can also be instantiated to represent the control-flow of any class that
+subclasses ``Stmt``, which includes simple expressions.  Control-flow graphs
+are especially useful for performing `flow- or path-sensitive
+<http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities>`_ program
+analyses on a given function.
+
+Basic Blocks
+^^^^^^^^^^^^
+
+Concretely, an instance of ``CFG`` is a collection of basic blocks.  Each basic
+block is an instance of ``CFGBlock``, which simply contains an ordered sequence
+of ``Stmt*`` (each referring to statements in the AST).  The ordering of
+statements within a block indicates unconditional flow of control from one
+statement to the next.  :ref:`Conditional control-flow
+<ConditionalControlFlow>` is represented using edges between basic blocks.  The
+statements within a given ``CFGBlock`` can be traversed using the
+``CFGBlock::*iterator`` interface.
+
+A ``CFG`` object owns the instances of ``CFGBlock`` within the control-flow
+graph it represents.  Each ``CFGBlock`` within a CFG is also uniquely numbered
+(accessible via ``CFGBlock::getBlockID()``).  Currently the number is based on
+the ordering the blocks were created, but no assumptions should be made on how
+``CFGBlocks`` are numbered other than their numbers are unique and that they
+are numbered from 0..N-1 (where N is the number of basic blocks in the CFG).
+
+Entry and Exit Blocks
+^^^^^^^^^^^^^^^^^^^^^
+
+Each instance of ``CFG`` contains two special blocks: an *entry* block
+(accessible via ``CFG::getEntry()``), which has no incoming edges, and an
+*exit* block (accessible via ``CFG::getExit()``), which has no outgoing edges.
+Neither block contains any statements, and they serve the role of providing a
+clear entrance and exit for a body of code such as a function body.  The
+presence of these empty blocks greatly simplifies the implementation of many
+analyses built on top of CFGs.
+
+.. _ConditionalControlFlow:
+
+Conditional Control-Flow
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Conditional control-flow (such as those induced by if-statements and loops) is
+represented as edges between ``CFGBlocks``.  Because different C language
+constructs can induce control-flow, each ``CFGBlock`` also records an extra
+``Stmt*`` that represents the *terminator* of the block.  A terminator is
+simply the statement that caused the control-flow, and is used to identify the
+nature of the conditional control-flow between blocks.  For example, in the
+case of an if-statement, the terminator refers to the ``IfStmt`` object in the
+AST that represented the given branch.
+
+To illustrate, consider the following code example:
+
+.. code-block:: c++
+
+  int foo(int x) {
+    x = x + 1;
+    if (x > 2)
+      x++;
+    else {
+      x += 2;
+      x *= 2;
+    }
+
+    return x;
+  }
+
+After invoking the parser+semantic analyzer on this code fragment, the AST of
+the body of ``foo`` is referenced by a single ``Stmt*``.  We can then construct
+an instance of ``CFG`` representing the control-flow graph of this function
+body by single call to a static class method:
+
+.. code-block:: c++
+
+  Stmt *FooBody = ...
+  CFG *FooCFG = CFG::buildCFG(FooBody);
+
+It is the responsibility of the caller of ``CFG::buildCFG`` to ``delete`` the
+returned ``CFG*`` when the CFG is no longer needed.
+
+Along with providing an interface to iterate over its ``CFGBlocks``, the
+``CFG`` class also provides methods that are useful for debugging and
+visualizing CFGs.  For example, the method ``CFG::dump()`` dumps a
+pretty-printed version of the CFG to standard error.  This is especially useful
+when one is using a debugger such as gdb.  For example, here is the output of
+``FooCFG->dump()``:
+
+.. code-block:: c++
+
+ [ B5 (ENTRY) ]
+    Predecessors (0):
+    Successors (1): B4
+
+ [ B4 ]
+    1: x = x + 1
+    2: (x > 2)
+    T: if [B4.2]
+    Predecessors (1): B5
+    Successors (2): B3 B2
+
+ [ B3 ]
+    1: x++
+    Predecessors (1): B4
+    Successors (1): B1
+
+ [ B2 ]
+    1: x += 2
+    2: x *= 2
+    Predecessors (1): B4
+    Successors (1): B1
+
+ [ B1 ]
+    1: return x;
+    Predecessors (2): B2 B3
+    Successors (1): B0
+
+ [ B0 (EXIT) ]
+    Predecessors (1): B1
+    Successors (0):
+
+For each block, the pretty-printed output displays for each block the number of
+*predecessor* blocks (blocks that have outgoing control-flow to the given
+block) and *successor* blocks (blocks that have control-flow that have incoming
+control-flow from the given block).  We can also clearly see the special entry
+and exit blocks at the beginning and end of the pretty-printed output.  For the
+entry block (block B5), the number of predecessor blocks is 0, while for the
+exit block (block B0) the number of successor blocks is 0.
+
+The most interesting block here is B4, whose outgoing control-flow represents
+the branching caused by the sole if-statement in ``foo``.  Of particular
+interest is the second statement in the block, ``(x > 2)``, and the terminator,
+printed as ``if [B4.2]``.  The second statement represents the evaluation of
+the condition of the if-statement, which occurs before the actual branching of
+control-flow.  Within the ``CFGBlock`` for B4, the ``Stmt*`` for the second
+statement refers to the actual expression in the AST for ``(x > 2)``.  Thus
+pointers to subclasses of ``Expr`` can appear in the list of statements in a
+block, and not just subclasses of ``Stmt`` that refer to proper C statements.
+
+The terminator of block B4 is a pointer to the ``IfStmt`` object in the AST.
+The pretty-printer outputs ``if [B4.2]`` because the condition expression of
+the if-statement has an actual place in the basic block, and thus the
+terminator is essentially *referring* to the expression that is the second
+statement of block B4 (i.e., B4.2).  In this manner, conditions for
+control-flow (which also includes conditions for loops and switch statements)
+are hoisted into the actual basic block.
+
+.. Implicit Control-Flow
+.. ^^^^^^^^^^^^^^^^^^^^^
+
+.. A key design principle of the ``CFG`` class was to not require any
+.. transformations to the AST in order to represent control-flow.  Thus the
+.. ``CFG`` does not perform any "lowering" of the statements in an AST: loops
+.. are not transformed into guarded gotos, short-circuit operations are not
+.. converted to a set of if-statements, and so on.
+
+Constant Folding in the Clang AST
+---------------------------------
+
+There are several places where constants and constant folding matter a lot to
+the Clang front-end.  First, in general, we prefer the AST to retain the source
+code as close to how the user wrote it as possible.  This means that if they
+wrote "``5+4``", we want to keep the addition and two constants in the AST, we
+don't want to fold to "``9``".  This means that constant folding in various
+ways turns into a tree walk that needs to handle the various cases.
+
+However, there are places in both C and C++ that require constants to be
+folded.  For example, the C standard defines what an "integer constant
+expression" (i-c-e) is with very precise and specific requirements.  The
+language then requires i-c-e's in a lot of places (for example, the size of a
+bitfield, the value for a case statement, etc).  For these, we have to be able
+to constant fold the constants, to do semantic checks (e.g., verify bitfield
+size is non-negative and that case statements aren't duplicated).  We aim for
+Clang to be very pedantic about this, diagnosing cases when the code does not
+use an i-c-e where one is required, but accepting the code unless running with
+``-pedantic-errors``.
+
+Things get a little bit more tricky when it comes to compatibility with
+real-world source code.  Specifically, GCC has historically accepted a huge
+superset of expressions as i-c-e's, and a lot of real world code depends on
+this unfortuate accident of history (including, e.g., the glibc system
+headers).  GCC accepts anything its "fold" optimizer is capable of reducing to
+an integer constant, which means that the definition of what it accepts changes
+as its optimizer does.  One example is that GCC accepts things like "``case
+X-X:``" even when ``X`` is a variable, because it can fold this to 0.
+
+Another issue are how constants interact with the extensions we support, such
+as ``__builtin_constant_p``, ``__builtin_inf``, ``__extension__`` and many
+others.  C99 obviously does not specify the semantics of any of these
+extensions, and the definition of i-c-e does not include them.  However, these
+extensions are often used in real code, and we have to have a way to reason
+about them.
+
+Finally, this is not just a problem for semantic analysis.  The code generator
+and other clients have to be able to fold constants (e.g., to initialize global
+variables) and has to handle a superset of what C99 allows.  Further, these
+clients can benefit from extended information.  For example, we know that
+"``foo() || 1``" always evaluates to ``true``, but we can't replace the
+expression with ``true`` because it has side effects.
+
+Implementation Approach
+^^^^^^^^^^^^^^^^^^^^^^^
+
+After trying several different approaches, we've finally converged on a design
+(Note, at the time of this writing, not all of this has been implemented,
+consider this a design goal!).  Our basic approach is to define a single
+recursive method evaluation method (``Expr::Evaluate``), which is implemented
+in ``AST/ExprConstant.cpp``.  Given an expression with "scalar" type (integer,
+fp, complex, or pointer) this method returns the following information:
+
+* Whether the expression is an integer constant expression, a general constant
+  that was folded but has no side effects, a general constant that was folded
+  but that does have side effects, or an uncomputable/unfoldable value.
+* If the expression was computable in any way, this method returns the
+  ``APValue`` for the result of the expression.
+* If the expression is not evaluatable at all, this method returns information
+  on one of the problems with the expression.  This includes a
+  ``SourceLocation`` for where the problem is, and a diagnostic ID that explains
+  the problem.  The diagnostic should have ``ERROR`` type.
+* If the expression is not an integer constant expression, this method returns
+  information on one of the problems with the expression.  This includes a
+  ``SourceLocation`` for where the problem is, and a diagnostic ID that
+  explains the problem.  The diagnostic should have ``EXTENSION`` type.
+
+This information gives various clients the flexibility that they want, and we
+will eventually have some helper methods for various extensions.  For example,
+``Sema`` should have a ``Sema::VerifyIntegerConstantExpression`` method, which
+calls ``Evaluate`` on the expression.  If the expression is not foldable, the
+error is emitted, and it would return ``true``.  If the expression is not an
+i-c-e, the ``EXTENSION`` diagnostic is emitted.  Finally it would return
+``false`` to indicate that the AST is OK.
+
+Other clients can use the information in other ways, for example, codegen can
+just use expressions that are foldable in any way.
+
+Extensions
+^^^^^^^^^^
+
+This section describes how some of the various extensions Clang supports
+interacts with constant evaluation:
+
+* ``__extension__``: The expression form of this extension causes any
+  evaluatable subexpression to be accepted as an integer constant expression.
+* ``__builtin_constant_p``: This returns true (as an integer constant
+  expression) if the operand evaluates to either a numeric value (that is, not
+  a pointer cast to integral type) of integral, enumeration, floating or
+  complex type, or if it evaluates to the address of the first character of a
+  string literal (possibly cast to some other type).  As a special case, if
+  ``__builtin_constant_p`` is the (potentially parenthesized) condition of a
+  conditional operator expression ("``?:``"), only the true side of the
+  conditional operator is considered, and it is evaluated with full constant
+  folding.
+* ``__builtin_choose_expr``: The condition is required to be an integer
+  constant expression, but we accept any constant as an "extension of an
+  extension".  This only evaluates one operand depending on which way the
+  condition evaluates.
+* ``__builtin_classify_type``: This always returns an integer constant
+  expression.
+* ``__builtin_inf, nan, ...``: These are treated just like a floating-point
+  literal.
+* ``__builtin_abs, copysign, ...``: These are constant folded as general
+  constant expressions.
+* ``__builtin_strlen`` and ``strlen``: These are constant folded as integer
+  constant expressions if the argument is a string literal.
+
+How to change Clang
+===================
+
+How to add an attribute
+-----------------------
+
+To add an attribute, you'll have to add it to the list of attributes, add it to
+the parsing phase, and look for it in the AST scan.
+`r124217 <http://llvm.org/viewvc/llvm-project?view=rev&revision=124217>`_
+has a good example of adding a warning attribute.
+
+(Beware that this hasn't been reviewed/fixed by the people who designed the
+attributes system yet.)
+
+
+``include/clang/Basic/Attr.td``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+First, add your attribute to the `include/clang/Basic/Attr.td file
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup>`_.
+
+Each attribute gets a ``def`` inheriting from ``Attr`` or one of its
+subclasses.  ``InheritableAttr`` means that the attribute also applies to
+subsequent declarations of the same name.
+
+``Spellings`` lists the strings that can appear in ``__attribute__((here))`` or
+``[[here]]``.  All such strings will be synonymous.  If you want to allow the
+``[[]]`` C++11 syntax, you have to define a list of ``Namespaces``, which will
+let users write ``[[namespace::spelling]]``.  Using the empty string for a
+namespace will allow users to write just the spelling with no "``::``".
+
+``Subjects`` restricts what kinds of AST node to which this attribute can
+appertain (roughly, attach).
+
+``Args`` names the arguments the attribute takes, in order.  If ``Args`` is
+``[StringArgument<"Arg1">, IntArgument<"Arg2">]`` then
+``__attribute__((myattribute("Hello", 3)))`` will be a valid use.
+
+Boilerplate
+^^^^^^^^^^^
+
+Write a new ``HandleYourAttr()`` function in `lib/Sema/SemaDeclAttr.cpp
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup>`_,
+and add a case to the switch in ``ProcessNonInheritableDeclAttr()`` or
+``ProcessInheritableDeclAttr()`` forwarding to it.
+
+If your attribute causes extra warnings to fire, define a ``DiagGroup`` in
+`include/clang/Basic/DiagnosticGroups.td
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup>`_
+named after the attribute's ``Spelling`` with "_"s replaced by "-"s.  If you're
+only defining one diagnostic, you can skip ``DiagnosticGroups.td`` and use
+``InGroup<DiagGroup<"your-attribute">>`` directly in `DiagnosticSemaKinds.td
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup>`_
+
+The meat of your attribute
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Find an appropriate place in Clang to do whatever your attribute needs to do.
+Check for the attribute's presence using ``Decl::getAttr<YourAttr>()``.
+
+Update the :doc:`LanguageExtensions` document to describe your new attribute.
+
+How to add an expression or statement
+-------------------------------------
+
+Expressions and statements are one of the most fundamental constructs within a
+compiler, because they interact with many different parts of the AST, semantic
+analysis, and IR generation.  Therefore, adding a new expression or statement
+kind into Clang requires some care.  The following list details the various
+places in Clang where an expression or statement needs to be introduced, along
+with patterns to follow to ensure that the new expression or statement works
+well across all of the C languages.  We focus on expressions, but statements
+are similar.
+
+#. Introduce parsing actions into the parser.  Recursive-descent parsing is
+   mostly self-explanatory, but there are a few things that are worth keeping
+   in mind:
+
+   * Keep as much source location information as possible! You'll want it later
+     to produce great diagnostics and support Clang's various features that map
+     between source code and the AST.
+   * Write tests for all of the "bad" parsing cases, to make sure your recovery
+     is good.  If you have matched delimiters (e.g., parentheses, square
+     brackets, etc.), use ``Parser::BalancedDelimiterTracker`` to give nice
+     diagnostics when things go wrong.
+
+#. Introduce semantic analysis actions into ``Sema``.  Semantic analysis should
+   always involve two functions: an ``ActOnXXX`` function that will be called
+   directly from the parser, and a ``BuildXXX`` function that performs the
+   actual semantic analysis and will (eventually!) build the AST node.  It's
+   fairly common for the ``ActOnCXX`` function to do very little (often just
+   some minor translation from the parser's representation to ``Sema``'s
+   representation of the same thing), but the separation is still important:
+   C++ template instantiation, for example, should always call the ``BuildXXX``
+   variant.  Several notes on semantic analysis before we get into construction
+   of the AST:
+
+   * Your expression probably involves some types and some subexpressions.
+     Make sure to fully check that those types, and the types of those
+     subexpressions, meet your expectations.  Add implicit conversions where
+     necessary to make sure that all of the types line up exactly the way you
+     want them.  Write extensive tests to check that you're getting good
+     diagnostics for mistakes and that you can use various forms of
+     subexpressions with your expression.
+   * When type-checking a type or subexpression, make sure to first check
+     whether the type is "dependent" (``Type::isDependentType()``) or whether a
+     subexpression is type-dependent (``Expr::isTypeDependent()``).  If any of
+     these return ``true``, then you're inside a template and you can't do much
+     type-checking now.  That's normal, and your AST node (when you get there)
+     will have to deal with this case.  At this point, you can write tests that
+     use your expression within templates, but don't try to instantiate the
+     templates.
+   * For each subexpression, be sure to call ``Sema::CheckPlaceholderExpr()``
+     to deal with "weird" expressions that don't behave well as subexpressions.
+     Then, determine whether you need to perform lvalue-to-rvalue conversions
+     (``Sema::DefaultLvalueConversions``) or the usual unary conversions
+     (``Sema::UsualUnaryConversions``), for places where the subexpression is
+     producing a value you intend to use.
+   * Your ``BuildXXX`` function will probably just return ``ExprError()`` at
+     this point, since you don't have an AST.  That's perfectly fine, and
+     shouldn't impact your testing.
+
+#. Introduce an AST node for your new expression.  This starts with declaring
+   the node in ``include/Basic/StmtNodes.td`` and creating a new class for your
+   expression in the appropriate ``include/AST/Expr*.h`` header.  It's best to
+   look at the class for a similar expression to get ideas, and there are some
+   specific things to watch for:
+
+   * If you need to allocate memory, use the ``ASTContext`` allocator to
+     allocate memory.  Never use raw ``malloc`` or ``new``, and never hold any
+     resources in an AST node, because the destructor of an AST node is never
+     called.
+   * Make sure that ``getSourceRange()`` covers the exact source range of your
+     expression.  This is needed for diagnostics and for IDE support.
+   * Make sure that ``children()`` visits all of the subexpressions.  This is
+     important for a number of features (e.g., IDE support, C++ variadic
+     templates).  If you have sub-types, you'll also need to visit those
+     sub-types in the ``RecursiveASTVisitor``.
+   * Add printing support (``StmtPrinter.cpp``) and dumping support
+     (``StmtDumper.cpp``) for your expression.
+   * Add profiling support (``StmtProfile.cpp``) for your AST node, noting the
+     distinguishing (non-source location) characteristics of an instance of
+     your expression.  Omitting this step will lead to hard-to-diagnose
+     failures regarding matching of template declarations.
+
+#. Teach semantic analysis to build your AST node.  At this point, you can wire
+   up your ``Sema::BuildXXX`` function to actually create your AST.  A few
+   things to check at this point:
+
+   * If your expression can construct a new C++ class or return a new
+     Objective-C object, be sure to update and then call
+     ``Sema::MaybeBindToTemporary`` for your just-created AST node to be sure
+     that the object gets properly destructed.  An easy way to test this is to
+     return a C++ class with a private destructor: semantic analysis should
+     flag an error here with the attempt to call the destructor.
+   * Inspect the generated AST by printing it using ``clang -cc1 -ast-print``,
+     to make sure you're capturing all of the important information about how
+     the AST was written.
+   * Inspect the generated AST under ``clang -cc1 -ast-dump`` to verify that
+     all of the types in the generated AST line up the way you want them.
+     Remember that clients of the AST should never have to "think" to
+     understand what's going on.  For example, all implicit conversions should
+     show up explicitly in the AST.
+   * Write tests that use your expression as a subexpression of other,
+     well-known expressions.  Can you call a function using your expression as
+     an argument?  Can you use the ternary operator?
+
+#. Teach code generation to create IR to your AST node.  This step is the first
+   (and only) that requires knowledge of LLVM IR.  There are several things to
+   keep in mind:
+
+   * Code generation is separated into scalar/aggregate/complex and
+     lvalue/rvalue paths, depending on what kind of result your expression
+     produces.  On occasion, this requires some careful factoring of code to
+     avoid duplication.
+   * ``CodeGenFunction`` contains functions ``ConvertType`` and
+     ``ConvertTypeForMem`` that convert Clang's types (``clang::Type*`` or
+     ``clang::QualType``) to LLVM types.  Use the former for values, and the
+     later for memory locations: test with the C++ "``bool``" type to check
+     this.  If you find that you are having to use LLVM bitcasts to make the
+     subexpressions of your expression have the type that your expression
+     expects, STOP!  Go fix semantic analysis and the AST so that you don't
+     need these bitcasts.
+   * The ``CodeGenFunction`` class has a number of helper functions to make
+     certain operations easy, such as generating code to produce an lvalue or
+     an rvalue, or to initialize a memory location with a given value.  Prefer
+     to use these functions rather than directly writing loads and stores,
+     because these functions take care of some of the tricky details for you
+     (e.g., for exceptions).
+   * If your expression requires some special behavior in the event of an
+     exception, look at the ``push*Cleanup`` functions in ``CodeGenFunction``
+     to introduce a cleanup.  You shouldn't have to deal with
+     exception-handling directly.
+   * Testing is extremely important in IR generation.  Use ``clang -cc1
+     -emit-llvm`` and `FileCheck
+     <http://llvm.org/docs/CommandGuide/FileCheck.html>`_ to verify that you're
+     generating the right IR.
+
+#. Teach template instantiation how to cope with your AST node, which requires
+   some fairly simple code:
+
+   * Make sure that your expression's constructor properly computes the flags
+     for type dependence (i.e., the type your expression produces can change
+     from one instantiation to the next), value dependence (i.e., the constant
+     value your expression produces can change from one instantiation to the
+     next), instantiation dependence (i.e., a template parameter occurs
+     anywhere in your expression), and whether your expression contains a
+     parameter pack (for variadic templates).  Often, computing these flags
+     just means combining the results from the various types and
+     subexpressions.
+   * Add ``TransformXXX`` and ``RebuildXXX`` functions to the ``TreeTransform``
+     class template in ``Sema``.  ``TransformXXX`` should (recursively)
+     transform all of the subexpressions and types within your expression,
+     using ``getDerived().TransformYYY``.  If all of the subexpressions and
+     types transform without error, it will then call the ``RebuildXXX``
+     function, which will in turn call ``getSema().BuildXXX`` to perform
+     semantic analysis and build your expression.
+   * To test template instantiation, take those tests you wrote to make sure
+     that you were type checking with type-dependent expressions and dependent
+     types (from step #2) and instantiate those templates with various types,
+     some of which type-check and some that don't, and test the error messages
+     in each case.
+
+#. There are some "extras" that make other features work better.  It's worth
+   handling these extras to give your expression complete integration into
+   Clang:
+
+   * Add code completion support for your expression in
+     ``SemaCodeComplete.cpp``.
+   * If your expression has types in it, or has any "interesting" features
+     other than subexpressions, extend libclang's ``CursorVisitor`` to provide
+     proper visitation for your expression, enabling various IDE features such
+     as syntax highlighting, cross-referencing, and so on.  The
+     ``c-index-test`` helper program can be used to test these features.
+