[ObjC] Implement @available in the Parser and AST

This patch adds a new AST node: ObjCAvailabilityCheckExpr, and teaches the
Parser and Sema to generate it. This node represents an availability check of
the form:

  @available(macos 10.10, *);

Which will eventually compile to a runtime check of the host's OS version. This
is the first patch of the feature I proposed here:
http://lists.llvm.org/pipermail/cfe-dev/2016-July/049851.html

Differential Revision: https://reviews.llvm.org/D22171

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275654 91177308-0d34-0410-b5e6-96231b3b80d8

30 files changed, 364 insertions, 5 deletions

diff --git a/include/clang-c/Index.h b/include/clang-c/Index.h
index ff3c86ff2e..13db2085ba 100644
--- a/include/clang-c/Index.h
+++ b/include/clang-c/Index.h
@@ -2014,7 +2014,11 @@ enum CXCursorKind {
    */
   CXCursor_OMPArraySectionExpr           = 147,
 
-  CXCursor_LastExpr                      = CXCursor_OMPArraySectionExpr,
+  /** \brief Represents an @available(...) check.
+   */
+  CXCursor_ObjCAvailabilityCheckExpr     = 148,
+
+  CXCursor_LastExpr                      = CXCursor_ObjCAvailabilityCheckExpr,
 
   /* Statements */
   CXCursor_FirstStmt                     = 200,
diff --git a/include/clang/AST/Availability.h b/include/clang/AST/Availability.h
new file mode 100644
index 0000000000..5ed8313784
--- /dev/null
+++ b/include/clang/AST/Availability.h
@@ -0,0 +1,63 @@
+//===--- Availability.h - Classes for availability --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This files defines some classes that implement availability checking.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_AVAILABILITY_H
+#define LLVM_CLANG_AST_AVAILABILITY_H
+
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Basic/VersionTuple.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace clang {
+
+/// \brief One specifier in an @available expression.
+///
+/// \code
+///   @available(macos 10.10, *)
+/// \endcode
+///
+/// Here, 'macos 10.10' and '*' both map to an instance of this type.
+///
+class AvailabilitySpec {
+  /// Represents the version that this specifier requires. If the host OS
+  /// version is greater than or equal to Version, the @available will evaluate
+  /// to true.
+  VersionTuple Version;
+
+  /// Name of the platform that Version corresponds to.
+  StringRef Platform;
+
+  SourceLocation BeginLoc, EndLoc;
+
+public:
+  AvailabilitySpec(VersionTuple Version, StringRef Platform,
+                   SourceLocation BeginLoc, SourceLocation EndLoc)
+      : Version(Version), Platform(Platform), BeginLoc(BeginLoc),
+        EndLoc(EndLoc) {}
+
+  /// This constructor is used when representing the '*' case.
+  AvailabilitySpec(SourceLocation StarLoc)
+      : BeginLoc(StarLoc), EndLoc(StarLoc) {}
+
+  VersionTuple getVersion() const { return Version; }
+  StringRef getPlatform() const { return Platform; }
+  SourceLocation getBeginLoc() const { return BeginLoc; }
+  SourceLocation getEndLoc() const { return EndLoc; }
+
+  /// Returns true when this represents the '*' case.
+  bool isOtherPlatformSpec() const { return Version.empty(); }
+};
+
+} // end namespace clang
+
+#endif
diff --git a/include/clang/AST/ExprObjC.h b/include/clang/AST/ExprObjC.h
index 61e6383bff..5f9623db24 100644
--- a/include/clang/AST/ExprObjC.h
+++ b/include/clang/AST/ExprObjC.h
@@ -1562,7 +1562,52 @@ public:
     return T->getStmtClass() == ObjCBridgedCastExprClass;
   }
 };
-  
+
+/// \brief A runtime availability query.
+///
+/// There are 2 ways to spell this node:
+/// \code
+///   @available(macos 10.10, ios 8, *); // Objective-C
+///   __builtin_available(macos 10.10, ios 8, *); // C, C++, and Objective-C
+/// \endcode
+///
+/// Note that we only need to keep track of one \c VersionTuple here, which is
+/// the one that corresponds to the current deployment target. This is meant to
+/// be used in the condition of an \c if, but it is also usable as top level
+/// expressions.
+///
+class ObjCAvailabilityCheckExpr : public Expr {
+  VersionTuple VersionToCheck;
+  SourceLocation AtLoc, RParen;
+
+  friend class ASTStmtReader;
+public:
+  ObjCAvailabilityCheckExpr(VersionTuple VersionToCheck, SourceLocation AtLoc,
+                            SourceLocation RParen, QualType Ty)
+      : Expr(ObjCAvailabilityCheckExprClass, Ty, VK_RValue, OK_Ordinary, false,
+             false, false, false),
+        VersionToCheck(VersionToCheck), AtLoc(AtLoc), RParen(RParen) {}
+
+  explicit ObjCAvailabilityCheckExpr(EmptyShell Shell)
+      : Expr(ObjCAvailabilityCheckExprClass, Shell) {}
+
+  SourceLocation getLocStart() const { return AtLoc; }
+  SourceLocation getLocEnd() const { return RParen; }
+  SourceRange getSourceRange() const { return {AtLoc, RParen}; }
+
+  /// \brief This may be '*', in which case this should fold to true.
+  bool hasVersion() const { return !VersionToCheck.empty(); }
+  VersionTuple getVersion() { return VersionToCheck; }
+
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == ObjCAvailabilityCheckExprClass;
+  }
+};
+
 }  // end namespace clang
 
 #endif
diff --git a/include/clang/AST/RecursiveASTVisitor.h b/include/clang/AST/RecursiveASTVisitor.h
index 94637a568c..f918b830d4 100644
--- a/include/clang/AST/RecursiveASTVisitor.h
+++ b/include/clang/AST/RecursiveASTVisitor.h
@@ -2352,6 +2352,7 @@ DEF_TRAVERSE_STMT(ObjCIndirectCopyRestoreExpr, {})
 DEF_TRAVERSE_STMT(ObjCBridgedCastExpr, {
   TRY_TO(TraverseTypeLoc(S->getTypeInfoAsWritten()->getTypeLoc()));
 })
+DEF_TRAVERSE_STMT(ObjCAvailabilityCheckExpr, {})
 DEF_TRAVERSE_STMT(ParenExpr, {})
 DEF_TRAVERSE_STMT(ParenListExpr, {})
 DEF_TRAVERSE_STMT(PredefinedExpr, {})
diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index 979c81f636..72dfedbbae 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -96,6 +96,7 @@ def DeprecatedAttributes : DiagGroup<"deprecated-attributes">;
 def DeprecatedDeclarations : DiagGroup<"deprecated-declarations">;
 def UnavailableDeclarations : DiagGroup<"unavailable-declarations">;
 def PartialAvailability : DiagGroup<"partial-availability">;
+def UnguardedAvailability : DiagGroup<"unguarded-availability">;
 def DeprecatedImplementations :DiagGroup<"deprecated-implementations">;
 def DeprecatedIncrementBool : DiagGroup<"deprecated-increment-bool">;
 def DeprecatedRegister : DiagGroup<"deprecated-register">;
diff --git a/include/clang/Basic/DiagnosticParseKinds.td b/include/clang/Basic/DiagnosticParseKinds.td
index e01d9463b3..e5c64681e4 100644
--- a/include/clang/Basic/DiagnosticParseKinds.td
+++ b/include/clang/Basic/DiagnosticParseKinds.td
@@ -823,6 +823,21 @@ def warn_availability_and_unavailable : Warning<
   "'unavailable' availability overrides all other availability information">,
   InGroup<Availability>;
 
+// @available(...)
+def err_avail_query_expected_condition : Error<
+  "expected an availability condition here">;
+def err_avail_query_expected_platform_name : Error<
+  "expected a platform name here">;
+
+def err_avail_query_unrecognized_platform_name : Error<
+  "unrecognized platform name %0">;
+def err_availability_query_wildcard_required: Error<
+  "must handle potential future platforms with '*'">;
+def err_availability_query_repeated_platform: Error<
+  "version for '%0' already specified">;
+def err_availability_query_repeated_star : Error<
+  "'*' query has already been specified">;
+
 // Type safety attributes
 def err_type_safety_unknown_flag : Error<
   "invalid comparison flag %0; use 'layout_compatible' or 'must_be_null'">;
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 45e77d84d4..1203fe765b 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2599,6 +2599,9 @@ def note_overridden_method : Note<
 def note_protocol_method : Note<
   "protocol method is here">;
 
+def warn_available_using_star_case : Warning<
+  "using '*' case here, platform %0 is not accounted for">, InGroup<UnguardedAvailability>;
+
 // Thread Safety Attributes
 def warn_invalid_capability_name : Warning<
   "invalid capability name '%0'; capability name must be 'mutex' or 'role'">,
diff --git a/include/clang/Basic/StmtNodes.td b/include/clang/Basic/StmtNodes.td
index df7e7362e2..973b3bb6fd 100644
--- a/include/clang/Basic/StmtNodes.td
+++ b/include/clang/Basic/StmtNodes.td
@@ -165,6 +165,7 @@ def ObjCIsaExpr : DStmt<Expr>;
 def ObjCIndirectCopyRestoreExpr : DStmt<Expr>;
 def ObjCBoolLiteralExpr : DStmt<Expr>;
 def ObjCSubscriptRefExpr : DStmt<Expr>;
+def ObjCAvailabilityCheckExpr : DStmt<Expr>;
 
 // Obj-C ARC Expressions.
 def ObjCBridgedCastExpr : DStmt<ExplicitCastExpr>;
diff --git a/include/clang/Basic/TokenKinds.def b/include/clang/Basic/TokenKinds.def
index a08d280e7e..882c6e4cac 100644
--- a/include/clang/Basic/TokenKinds.def
+++ b/include/clang/Basic/TokenKinds.def
@@ -630,6 +630,8 @@ KEYWORD(__builtin_convertvector   , KEYALL)
 ALIAS("__char16_t"   , char16_t   , KEYCXX)
 ALIAS("__char32_t"   , char32_t   , KEYCXX)
 
+KEYWORD(__builtin_available       , KEYALL)
+
 // Clang-specific keywords enabled only in testing.
 TESTING_KEYWORD(__unknown_anytype , KEYALL)
 
@@ -668,6 +670,7 @@ OBJC2_AT_KEYWORD(optional)
 OBJC2_AT_KEYWORD(synthesize)
 OBJC2_AT_KEYWORD(dynamic)
 OBJC2_AT_KEYWORD(import)
+OBJC2_AT_KEYWORD(available)
 
 // TODO: What to do about context-sensitive keywords like:
 //       bycopy/byref/in/inout/oneway/out?
diff --git a/include/clang/Parse/Parser.h b/include/clang/Parse/Parser.h
index 9d59f257af..5838a447c3 100644
--- a/include/clang/Parse/Parser.h
+++ b/include/clang/Parse/Parser.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_PARSE_PARSER_H
 #define LLVM_CLANG_PARSE_PARSER_H
 
+#include "clang/AST/Availability.h"
 #include "clang/Basic/OpenMPKinds.h"
 #include "clang/Basic/OperatorPrecedence.h"
 #include "clang/Basic/Specifiers.h"
@@ -2244,6 +2245,9 @@ private:
                                   SourceLocation ScopeLoc,
                                   AttributeList::Syntax Syntax);
 
+  Optional<AvailabilitySpec> ParseAvailabilitySpec();
+  ExprResult ParseAvailabilityCheckExpr(SourceLocation StartLoc);
+
   void ParseObjCBridgeRelatedAttribute(IdentifierInfo &ObjCBridgeRelated,
                                        SourceLocation ObjCBridgeRelatedLoc,
                                        ParsedAttributes &attrs,
diff --git a/include/clang/Sema/Sema.h b/include/clang/Sema/Sema.h
index 4f2f7300ae..25aa805dac 100644
--- a/include/clang/Sema/Sema.h
+++ b/include/clang/Sema/Sema.h
@@ -16,6 +16,7 @@
 #define LLVM_CLANG_SEMA_SEMA_H
 
 #include "clang/AST/Attr.h"
+#include "clang/AST/Availability.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprObjC.h"
@@ -4739,6 +4740,10 @@ public:
   /// ActOnObjCBoolLiteral - Parse {__objc_yes,__objc_no} literals.
   ExprResult ActOnObjCBoolLiteral(SourceLocation OpLoc, tok::TokenKind Kind);
 
+  ExprResult
+  ActOnObjCAvailabilityCheckExpr(llvm::ArrayRef<AvailabilitySpec> AvailSpecs,
+                                 SourceLocation AtLoc, SourceLocation RParen);
+
   /// ActOnCXXNullPtrLiteral - Parse 'nullptr'.
   ExprResult ActOnCXXNullPtrLiteral(SourceLocation Loc);
 
diff --git a/include/clang/Serialization/ASTBitCodes.h b/include/clang/Serialization/ASTBitCodes.h
index 809dbc8457..79c6a06222 100644
--- a/include/clang/Serialization/ASTBitCodes.h
+++ b/include/clang/Serialization/ASTBitCodes.h
@@ -1349,8 +1349,10 @@ namespace clang {
       STMT_OBJC_AT_THROW,
       /// \brief An ObjCAutoreleasePoolStmt record.
       STMT_OBJC_AUTORELEASE_POOL,
-      /// \brief A ObjCBoolLiteralExpr record.
+      /// \brief An ObjCBoolLiteralExpr record.
       EXPR_OBJC_BOOL_LITERAL,
+      /// \brief An ObjCAvailabilityCheckExpr record.
+      EXPR_OBJC_AVAILABILITY_CHECK,
 
       // C++
       
diff --git a/lib/AST/Expr.cpp b/lib/AST/Expr.cpp
index bd4cf396bf..091e8787d8 100644
--- a/lib/AST/Expr.cpp
+++ b/lib/AST/Expr.cpp
@@ -2856,6 +2856,7 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case ObjCStringLiteralClass:
   case ObjCEncodeExprClass:
   case ObjCBoolLiteralExprClass:
+  case ObjCAvailabilityCheckExprClass:
   case CXXUuidofExprClass:
   case OpaqueValueExprClass:
     // These never have a side-effect.
diff --git a/lib/AST/ExprClassification.cpp b/lib/AST/ExprClassification.cpp
index 642cdd1a95..89cc9bc18e 100644
--- a/lib/AST/ExprClassification.cpp
+++ b/lib/AST/ExprClassification.cpp
@@ -178,6 +178,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::ObjCArrayLiteralClass:
   case Expr::ObjCDictionaryLiteralClass:
   case Expr::ObjCBoolLiteralExprClass:
+  case Expr::ObjCAvailabilityCheckExprClass:
   case Expr::ParenListExprClass:
   case Expr::SizeOfPackExprClass:
   case Expr::SubstNonTypeTemplateParmPackExprClass:
diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index 28bdf6da3a..a25a6c7e51 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -9402,6 +9402,7 @@ static ICEDiag CheckICE(const Expr* E, const ASTContext &Ctx) {
   case Expr::ObjCPropertyRefExprClass:
   case Expr::ObjCSubscriptRefExprClass:
   case Expr::ObjCIsaExprClass:
+  case Expr::ObjCAvailabilityCheckExprClass:
   case Expr::ShuffleVectorExprClass:
   case Expr::ConvertVectorExprClass:
   case Expr::BlockExprClass:
diff --git a/lib/AST/ItaniumMangle.cpp b/lib/AST/ItaniumMangle.cpp
index 5a7c45594e..694fde3175 100644
--- a/lib/AST/ItaniumMangle.cpp
+++ b/lib/AST/ItaniumMangle.cpp
@@ -3239,6 +3239,7 @@ recurse:
   case Expr::ObjCDictionaryLiteralClass:
   case Expr::ObjCSubscriptRefExprClass:
   case Expr::ObjCIndirectCopyRestoreExprClass:
+  case Expr::ObjCAvailabilityCheckExprClass:
   case Expr::OffsetOfExprClass:
   case Expr::PredefinedExprClass:
   case Expr::ShuffleVectorExprClass:
diff --git a/lib/AST/StmtPrinter.cpp b/lib/AST/StmtPrinter.cpp
index 64d9a32956..8797a13335 100644
--- a/lib/AST/StmtPrinter.cpp
+++ b/lib/AST/StmtPrinter.cpp
@@ -497,6 +497,11 @@ void StmtPrinter::VisitObjCAtThrowStmt(ObjCAtThrowStmt *Node) {
   OS << ";\n";
 }
 
+void StmtPrinter::VisitObjCAvailabilityCheckExpr(
+    ObjCAvailabilityCheckExpr *Node) {
+  OS << "@available(...)";
+}
+
 void StmtPrinter::VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt *Node) {
   Indent() << "@synchronized (";
   PrintExpr(Node->getSynchExpr());
diff --git a/lib/AST/StmtProfile.cpp b/lib/AST/StmtProfile.cpp
index 5ce6e5e13a..0a39413853 100644
--- a/lib/AST/StmtProfile.cpp
+++ b/lib/AST/StmtProfile.cpp
@@ -1632,6 +1632,11 @@ void StmtProfiler::VisitObjCBridgedCastExpr(const ObjCBridgedCastExpr *S) {
   ID.AddBoolean(S->getBridgeKind());
 }
 
+void StmtProfiler::VisitObjCAvailabilityCheckExpr(
+    const ObjCAvailabilityCheckExpr *S) {
+  VisitExpr(S);
+}
+
 void StmtProfiler::VisitDecl(const Decl *D) {
   ID.AddInteger(D? D->getKind() : 0);
 
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 9a754ee23c..45e1c3e465 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -720,7 +720,7 @@ static bool VersionNumberSeparator(const char Separator) {
 ///   simple-integer ',' simple-integer
 ///   simple-integer ',' simple-integer ',' simple-integer
 VersionTuple Parser::ParseVersionTuple(SourceRange &Range) {
-  Range = Tok.getLocation();
+  Range = SourceRange(Tok.getLocation(), Tok.getEndLoc());
 
   if (!Tok.is(tok::numeric_constant)) {
     Diag(Tok, diag::err_expected_version);
diff --git a/lib/Parse/ParseExpr.cpp b/lib/Parse/ParseExpr.cpp
index 4e39d42ea6..47d1326b79 100644
--- a/lib/Parse/ParseExpr.cpp
+++ b/lib/Parse/ParseExpr.cpp
@@ -1009,6 +1009,8 @@ ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
   case tok::kw__Generic:   // primary-expression: generic-selection [C11 6.5.1]
     Res = ParseGenericSelectionExpression();
     break;
+  case tok::kw___builtin_available:
+    return ParseAvailabilityCheckExpr(Tok.getLocation());
   case tok::kw___builtin_va_arg:
   case tok::kw___builtin_offsetof:
   case tok::kw___builtin_choose_expr:
@@ -2869,3 +2871,117 @@ ExprResult Parser::ParseObjCBoolLiteral() {
   tok::TokenKind Kind = Tok.getKind();
   return Actions.ActOnObjCBoolLiteral(ConsumeToken(), Kind);
 }
+
+/// Validate availability spec list, emitting diagnostics if necessary. Returns
+/// true if invalid.
+static bool CheckAvailabilitySpecList(Parser &P,
+                                      ArrayRef<AvailabilitySpec> AvailSpecs) {
+  llvm::SmallSet<StringRef, 4> Platforms;
+  bool HasOtherPlatformSpec = false;
+  bool Valid = true;
+  for (const auto &Spec : AvailSpecs) {
+    if (Spec.isOtherPlatformSpec()) {
+      if (HasOtherPlatformSpec) {
+        P.Diag(Spec.getBeginLoc(), diag::err_availability_query_repeated_star);
+        Valid = false;
+      }
+
+      HasOtherPlatformSpec = true;
+      continue;
+    }
+
+    bool Inserted = Platforms.insert(Spec.getPlatform()).second;
+    if (!Inserted) {
+      // Rule out multiple version specs referring to the same platform.
+      // For example, we emit an error for:
+      // @available(macos 10.10, macos 10.11, *)
+      StringRef Platform = Spec.getPlatform();
+      P.Diag(Spec.getBeginLoc(), diag::err_availability_query_repeated_platform)
+          << Spec.getEndLoc() << Platform;
+      Valid = false;
+    }
+  }
+
+  if (!HasOtherPlatformSpec) {
+    SourceLocation InsertWildcardLoc = AvailSpecs.back().getEndLoc();
+    P.Diag(InsertWildcardLoc, diag::err_availability_query_wildcard_required)
+        << FixItHint::CreateInsertion(InsertWildcardLoc, ", *");
+    return true;
+  }
+
+  return !Valid;
+}
+
+/// Parse availability query specification.
+///
+///  availability-spec:
+///     '*'
+///     identifier version-tuple
+Optional<AvailabilitySpec> Parser::ParseAvailabilitySpec() {
+  if (Tok.is(tok::star)) {
+    return AvailabilitySpec(ConsumeToken());
+  } else {
+    // Parse the platform name.
+    if (Tok.isNot(tok::identifier)) {
+      Diag(Tok, diag::err_avail_query_expected_platform_name);
+      return None;
+    }
+
+    IdentifierLoc *PlatformIdentifier = ParseIdentifierLoc();
+    SourceRange VersionRange;
+    VersionTuple Version = ParseVersionTuple(VersionRange);
+
+    if (Version.empty())
+      return None;
+
+    StringRef Platform = PlatformIdentifier->Ident->getName();
+
+    if (AvailabilityAttr::getPrettyPlatformName(Platform).empty()) {
+      Diag(PlatformIdentifier->Loc,
+           diag::err_avail_query_unrecognized_platform_name)
+          << Platform;
+      return None;
+    }
+
+    return AvailabilitySpec(Version, Platform, PlatformIdentifier->Loc,
+                            VersionRange.getEnd());
+  }
+}
+
+ExprResult Parser::ParseAvailabilityCheckExpr(SourceLocation BeginLoc) {
+  assert(Tok.is(tok::kw___builtin_available) ||
+         Tok.isObjCAtKeyword(tok::objc_available));
+
+  // Eat the available or __builtin_available.
+  ConsumeToken();
+
+  BalancedDelimiterTracker Parens(*this, tok::l_paren);
+  if (Parens.expectAndConsume())
+    return ExprError();
+
+  SmallVector<AvailabilitySpec, 4> AvailSpecs;
+  bool HasError = false;
+  while (true) {
+    Optional<AvailabilitySpec> Spec = ParseAvailabilitySpec();
+    if (!Spec)
+      HasError = true;
+    else
+      AvailSpecs.push_back(*Spec);
+
+    if (!TryConsumeToken(tok::comma))
+      break;
+  }
+
+  if (HasError) {
+    SkipUntil(tok::r_paren, StopAtSemi);
+    return ExprError();
+  }
+
+  CheckAvailabilitySpecList(*this, AvailSpecs);
+
+  if (Parens.consumeClose())
+    return ExprError();
+
+  return Actions.ActOnObjCAvailabilityCheckExpr(AvailSpecs, BeginLoc,
+                                                Parens.getCloseLocation());
+}
diff --git a/lib/Parse/ParseObjc.cpp b/lib/Parse/ParseObjc.cpp
index 22c832650b..67abe5839b 100644
--- a/lib/Parse/ParseObjc.cpp
+++ b/lib/Parse/ParseObjc.cpp
@@ -2858,6 +2858,8 @@ ExprResult Parser::ParseObjCAtExpression(SourceLocation AtLoc) {
       return ParsePostfixExpressionSuffix(ParseObjCProtocolExpression(AtLoc));
     case tok::objc_selector:
       return ParsePostfixExpressionSuffix(ParseObjCSelectorExpression(AtLoc));
+    case tok::objc_available:
+      return ParseAvailabilityCheckExpr(AtLoc);
       default: {
         const char *str = nullptr;
         if (GetLookAheadToken(1).is(tok::l_brace)) {
diff --git a/lib/Sema/SemaExceptionSpec.cpp b/lib/Sema/SemaExceptionSpec.cpp
index 61ea87aa9d..4a21eb308f 100644
--- a/lib/Sema/SemaExceptionSpec.cpp
+++ b/lib/Sema/SemaExceptionSpec.cpp
@@ -1146,6 +1146,7 @@ CanThrowResult Sema::canThrow(const Expr *E) {
   case Expr::ObjCIndirectCopyRestoreExprClass:
   case Expr::ObjCProtocolExprClass:
   case Expr::ObjCSelectorExprClass:
+  case Expr::ObjCAvailabilityCheckExprClass:
   case Expr::OffsetOfExprClass:
   case Expr::PackExpansionExprClass:
   case Expr::PseudoObjectExprClass:
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 0c8a913694..57159fb597 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -15083,3 +15083,27 @@ Sema::ActOnObjCBoolLiteral(SourceLocation OpLoc, tok::TokenKind Kind) {
   return new (Context)
       ObjCBoolLiteralExpr(Kind == tok::kw___objc_yes, BoolT, OpLoc);
 }
+
+ExprResult Sema::ActOnObjCAvailabilityCheckExpr(
+    llvm::ArrayRef<AvailabilitySpec> AvailSpecs, SourceLocation AtLoc,
+    SourceLocation RParen) {
+
+  StringRef Platform = getASTContext().getTargetInfo().getPlatformName();
+
+  auto Spec = std::find_if(AvailSpecs.begin(), AvailSpecs.end(),
+                           [&](const AvailabilitySpec &Spec) {
+                             return Spec.getPlatform() == Platform;
+                           });
+
+  VersionTuple Version;
+  if (Spec != AvailSpecs.end())
+    Version = Spec->getVersion();
+  else
+    // This is the '*' case in @available. We should diagnose this; the
+    // programmer should explicitly account for this case if they target this
+    // platform.
+    Diag(AtLoc, diag::warn_available_using_star_case) << RParen << Platform;
+
+  return new (Context)
+      ObjCAvailabilityCheckExpr(Version, AtLoc, RParen, Context.BoolTy);
+}
diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index 0dbdebf5db..36859f61d3 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -11112,6 +11112,12 @@ TransformObjCBridgedCastExpr(ObjCBridgedCastExpr *E) {
                                       Result.get());
 }
 
+template <typename Derived>
+ExprResult TreeTransform<Derived>::TransformObjCAvailabilityCheckExpr(
+    ObjCAvailabilityCheckExpr *E) {
+  return E;
+}
+
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformObjCMessageExpr(ObjCMessageExpr *E) {
diff --git a/lib/Serialization/ASTReaderStmt.cpp b/lib/Serialization/ASTReaderStmt.cpp
index 2fa3a4400b..395da42d4f 100644
--- a/lib/Serialization/ASTReaderStmt.cpp
+++ b/lib/Serialization/ASTReaderStmt.cpp
@@ -1182,6 +1182,14 @@ void ASTStmtReader::VisitObjCBoolLiteralExpr(ObjCBoolLiteralExpr *E) {
   E->setLocation(ReadSourceLocation(Record, Idx));
 }
 
+void ASTStmtReader::VisitObjCAvailabilityCheckExpr(ObjCAvailabilityCheckExpr *E) {
+  VisitExpr(E);
+  SourceRange R = Reader.ReadSourceRange(F, Record, Idx);
+  E->AtLoc = R.getBegin();
+  E->RParen = R.getEnd();
+  E->VersionToCheck = Reader.ReadVersionTuple(Record, Idx);
+}
+
 //===----------------------------------------------------------------------===//
 // C++ Expressions and Statements
 //===----------------------------------------------------------------------===//
@@ -3221,6 +3229,9 @@ Stmt *ASTReader::ReadStmtFromStream(ModuleFile &F) {
     case EXPR_OBJC_BOOL_LITERAL:
       S = new (Context) ObjCBoolLiteralExpr(Empty);
       break;
+    case EXPR_OBJC_AVAILABILITY_CHECK:
+      S = new (Context) ObjCAvailabilityCheckExpr(Empty);
+      break;
     case STMT_SEH_LEAVE:
       S = new (Context) SEHLeaveStmt(Empty);
       break;
diff --git a/lib/Serialization/ASTWriterStmt.cpp b/lib/Serialization/ASTWriterStmt.cpp
index 73f7219ba5..84e718e9ef 100644
--- a/lib/Serialization/ASTWriterStmt.cpp
+++ b/lib/Serialization/ASTWriterStmt.cpp
@@ -1142,6 +1142,13 @@ void ASTStmtWriter::VisitObjCBoolLiteralExpr(ObjCBoolLiteralExpr *E) {
   Code = serialization::EXPR_OBJC_BOOL_LITERAL;
 }
 
+void ASTStmtWriter::VisitObjCAvailabilityCheckExpr(ObjCAvailabilityCheckExpr *E) {
+  VisitExpr(E);
+  Record.AddSourceRange(E->getSourceRange());
+  Record.AddVersionTuple(E->getVersion());
+  Code = serialization::EXPR_OBJC_AVAILABILITY_CHECK;
+}
+
 //===----------------------------------------------------------------------===//
 // C++ Expressions and Statements.
 //===----------------------------------------------------------------------===//
diff --git a/lib/StaticAnalyzer/Core/ExprEngine.cpp b/lib/StaticAnalyzer/Core/ExprEngine.cpp
index baa03e0e51..405aecdee0 100644
--- a/lib/StaticAnalyzer/Core/ExprEngine.cpp
+++ b/lib/StaticAnalyzer/Core/ExprEngine.cpp
@@ -916,6 +916,7 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode *Pred,
     case Stmt::CXXScalarValueInitExprClass:
     case Stmt::CXXBoolLiteralExprClass:
     case Stmt::ObjCBoolLiteralExprClass:
+    case Stmt::ObjCAvailabilityCheckExprClass:
     case Stmt::FloatingLiteralClass:
     case Stmt::NoInitExprClass:
     case Stmt::SizeOfPackExprClass:
diff --git a/test/Parser/objc-available.m b/test/Parser/objc-available.m
new file mode 100644
index 0000000000..ca30753144
--- /dev/null
+++ b/test/Parser/objc-available.m
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -fsyntax-only -Wunguarded-availability -triple x86_64-apple-macosx10.10.0 -verify %s
+
+void f() {
+
+  if (@available(macos 10.12, *)) {}
+  else if (@available(macos 10.11, *)) {}
+  else {}
+
+  (void)__builtin_available(ios 8, macos 10.10, *);
+
+  (void)@available(macos 10.11); // expected-error{{must handle potential future platforms with '*'}}
+  (void)@available(macos 10.11, macos 10.11, *); // expected-error{{version for 'macos' already specified}}
+
+  (void)@available(erik_os 10.11, *); // expected-error{{unrecognized platform name erik_os}}
+
+  (void)@available(erik_os 10.10, hat_os 1.0, *); // expected-error 2 {{unrecognized platform name}}
+
+  (void)@available(ios 8, *); // expected-warning{{using '*' case here, platform macos is not accounted for}}
+
+  (void)@available(); // expected-error{{expected a platform name here}}
+  (void)@available(macos 10.10,); // expected-error{{expected a platform name here}}
+  (void)@available(macos); // expected-error{{expected a version}}
+  (void)@available; // expected-error{{expected '('}}
+}
diff --git a/tools/libclang/CIndex.cpp b/tools/libclang/CIndex.cpp
index f4f9c16225..027bf95b66 100644
--- a/tools/libclang/CIndex.cpp
+++ b/tools/libclang/CIndex.cpp
@@ -4611,6 +4611,8 @@ CXString clang_getCursorKindSpelling(enum CXCursorKind Kind) {
       return cxstring::createRef("ObjCStringLiteral");
   case CXCursor_ObjCBoolLiteralExpr:
       return cxstring::createRef("ObjCBoolLiteralExpr");
+  case CXCursor_ObjCAvailabilityCheckExpr:
+      return cxstring::createRef("ObjCAvailabilityCheckExpr");
   case CXCursor_ObjCSelfExpr:
       return cxstring::createRef("ObjCSelfExpr");
   case CXCursor_ObjCEncodeExpr:
diff --git a/tools/libclang/CXCursor.cpp b/tools/libclang/CXCursor.cpp
index 80bb569150..19ab0f929c 100644
--- a/tools/libclang/CXCursor.cpp
+++ b/tools/libclang/CXCursor.cpp
@@ -447,7 +447,11 @@ CXCursor cxcursor::MakeCXCursor(const Stmt *S, const Decl *Parent,
   case Stmt::ObjCBoolLiteralExprClass:
     K = CXCursor_ObjCBoolLiteralExpr;
     break;
-      
+
+  case Stmt::ObjCAvailabilityCheckExprClass:
+    K = CXCursor_ObjCAvailabilityCheckExpr;
+    break;
+
   case Stmt::ObjCBridgedCastExprClass:
     K = CXCursor_ObjCBridgedCastExpr;
     break;