[clang-tidy] Improve alternate snake case warnings (#71385)

Improves the accuracy of `readability-identifier-naming` for cases
`Camel_Snake_Case` and `camel_Snake_Back`. Prior to this commit, these
cases matched identifiers with **only** a leading upper case letter or
leading lower case letter respectively. Now, uppercase letters can only
appear at the start of an identifier or directly following an
underscore.

---

Currently, the regex for `Camel_Snake_Case` matches any identifier that
starts with a capital letter:

```
^[A-Z]([a-z0-9]*(_[A-Z])?)*
                ^^^^^^^^^-- underscore + capital letter after the first capital is optional
```

This means that `Camel_Snake_Case` matches other cases - in particular
`CamelCase` and `Leading_upper_snake_case` - which causes clang-tidy to
sometimes not flag incorrect casing. It also matches `UPPER_CASE`, but I
think it's reasonable to consider this a subset of `Camel_Snake_Case`
since some users may prefer e.g. `XML_Parser` to `Xml_Parser`. It's
really easy to accidentally type an identifier that clang-tidy doesn't
catch; all you have to do is omit an underscore or forget to capitalize
a letter. The same problem also applies to `camel_Snake_Back` except
that any identifier starting with a lower case letter matches, so I went
ahead and adjusted its regex too. Fixing it also uncovered a minor error
in an existing test.

4 files changed, 66 insertions, 4 deletions

diff --git a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
index 066057fa7208..18c5e144e46f 100644
--- a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
@@ -872,8 +872,8 @@ bool IdentifierNamingCheck::matchesStyle(
       llvm::Regex("^[a-z][a-zA-Z0-9]*$"),
       llvm::Regex("^[A-Z][A-Z0-9_]*$"),
       llvm::Regex("^[A-Z][a-zA-Z0-9]*$"),
-      llvm::Regex("^[A-Z]([a-z0-9]*(_[A-Z])?)*"),
-      llvm::Regex("^[a-z]([a-z0-9]*(_[A-Z])?)*"),
+      llvm::Regex("^[A-Z]+([a-z0-9]*_[A-Z0-9]+)*[a-z0-9]*$"),
+      llvm::Regex("^[a-z]+([a-z0-9]*_[A-Z0-9]+)*[a-z0-9]*$"),
       llvm::Regex("^[A-Z]([a-z0-9_]*[a-z])*$"),
   };
 
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 23111be4371e..6d5f49dc0625 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -416,7 +416,8 @@ Changes in existing checks
   ``Leading_upper_snake_case`` naming convention. The handling of ``typedef``
   has been enhanced, particularly within complex types like function pointers
   and cases where style checks were omitted when functions started with macros.
-  Added support for C++20 ``concept`` declarations.
+  Added support for C++20 ``concept`` declarations. ``Camel_Snake_Case`` and
+  ``camel_Snake_Case`` now detect more invalid identifier names.
 
 - Improved :doc:`readability-implicit-bool-conversion
   <clang-tidy/checks/readability/implicit-bool-conversion>` check to take
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-case-match.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-case-match.cpp
new file mode 100644
index 000000000000..f692b0192345
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-case-match.cpp
@@ -0,0 +1,60 @@
+// RUN: %check_clang_tidy -std=c++20 %s readability-identifier-naming %t -- \
+// RUN:   -config='{CheckOptions: { \
+// RUN:     readability-identifier-naming.ClassCase: Camel_Snake_Case, \
+// RUN:     readability-identifier-naming.StructCase: camel_Snake_Back, \
+// RUN:   }}'
+
+// clang-format off
+
+//===----------------------------------------------------------------------===//
+// Camel_Snake_Case tests
+//===----------------------------------------------------------------------===//
+class XML_Parser {};
+class Xml_Parser {};
+class XML_Parser_2 {};
+// NO warnings or fixes expected as these identifiers are Camel_Snake_Case
+
+class XmlParser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'XmlParser'
+// CHECK-FIXES: {{^}}class Xml_Parser {};{{$}}
+
+class Xml_parser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'Xml_parser'
+// CHECK-FIXES: {{^}}class Xml_Parser {};{{$}}
+
+class xml_parser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'xml_parser'
+// CHECK-FIXES: {{^}}class Xml_Parser {};{{$}}
+
+class xml_Parser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'xml_Parser'
+// CHECK-FIXES: {{^}}class Xml_Parser {};{{$}}
+
+class xml_Parser_2 {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'xml_Parser_2'
+// CHECK-FIXES: {{^}}class Xml_Parser_2 {};{{$}}
+
+class t {};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 't'
+// CHECK-FIXES: {{^}}class T {};{{$}}
+
+//===----------------------------------------------------------------------===//
+// camel_Snake_Back tests
+//===----------------------------------------------------------------------===//
+struct json_Parser {};
+struct json_Parser_2 {};
+struct u {};
+// NO warnings or fixes expected as these identifiers are camel_Snake_Back
+
+struct JsonParser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: invalid case style for struct 'JsonParser'
+// CHECK-FIXES: {{^}}struct json_Parser {};{{$}}
+
+struct Json_parser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: invalid case style for struct 'Json_parser'
+// CHECK-FIXES: {{^}}struct json_Parser {};{{$}}
+
+struct json_parser {};
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: invalid case style for struct 'json_parser'
+// CHECK-FIXES: {{^}}struct json_Parser {};{{$}}
+
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp
index e375aa098972..d2e89a7c9855 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp
@@ -423,7 +423,8 @@ class my_other_templated_class : my_templated_class<  my_class>, private my_deri
 
 template<typename t_t>
 using mysuper_tpl_t = my_other_templated_class  <:: FOO_NS  ::my_class>;
-// CHECK-FIXES: {{^}}using mysuper_tpl_t = CMyOtherTemplatedClass  <:: foo_ns  ::CMyClass>;{{$}}
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for type alias 'mysuper_tpl_t'
+// CHECK-FIXES: {{^}}using mysuper_Tpl_t = CMyOtherTemplatedClass  <:: foo_ns  ::CMyClass>;{{$}}
 
 const int global_Constant = 6;
 // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: invalid case style for global constant 'global_Constant'