[clang-tidy] Fixed false-negative in readability-identifier-naming

[PiotrZSL]

Summary

Issue were cased by checking if canonical declaration of record is a
definition. Unfortunately when forward declarations were used, it were
not a definition.

Fixes: #64463

[PiotrZSL]

Properly handle situation when class is forward-declared as struct.

[carlosgalvezp]

LGTM, minor question that can be fixed post-review unless you want to discuss further!

clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp

716

Nit: shouldn't we use also struct here?

[PiotrZSL]

clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp

716

No, thats on prupose. I wanted to test also in same place that style is taken from definition, not forward declaration, can add some comment, and "public:"

[PiotrZSL]

Rebase + make tests more simple

clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp

if (const auto *EnumConst = dyn_cast<EnumConstantDecl>(D)) {
 
     if (NamingStyles[SK_Constant])
       return SK_Constant;
 
     return SK_Invalid;
   }
 
   if (const auto *Decl = dyn_cast<CXXRecordDecl>(D)) {
-    if (Decl->isAnonymousStructOrUnion())
+    if (Decl->isAnonymousStructOrUnion() || !Decl->hasDefinition())
       return SK_Invalid;
 
-    if (!Decl->getCanonicalDecl()->isThisDeclarationADefinition())
-      return SK_Invalid;
-
-    if (Decl->hasDefinition() && Decl->isAbstract() &&
-        NamingStyles[SK_AbstractClass])
+    if (Decl->isAbstract() && NamingStyles[SK_AbstractClass])
       return SK_AbstractClass;
 
     if (Decl->isStruct() && NamingStyles[SK_Struct])
       return SK_Struct;
 
     if (Decl->isStruct() && NamingStyles[SK_Class])
       return SK_Class;
 

clang-tools-extra/docs/ReleaseNotes.rst

 
 Changes in existing checks
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 - Fixed bug in :doc:`bugprone-reserved-identifier
   <clang-tidy/checks/bugprone/reserved-identifier>`, so that it does not warn
   on macros starting with underscore and lowercase letter.
 
+- Improved the :doc:`readability-identifier-naming
+  <clang-tidy/checks/readability/identifier-naming>` check to emit proper
+  warnings when a type forward declaration precedes its definition.
+
 Removed checks
 ^^^^^^^^^^^^^^
 
 Improvements to include-fixer
 -----------------------------
 
 The improvements are...
 

clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming.cpp

 struct async_obj {
 public:
   never_suspend operator co_await() const noexcept;
 };
 
 task ImplicitDeclTest(async_obj &a_object) {
   co_await a_object;  // CHECK-MESSAGES-NOT: warning: invalid case style for local variable
 }
+
+// Test scenario when canonical declaration will be a forward declaration
+class class_with_forward_decl;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'class_with_forward_decl' [readability-identifier-naming]
+// CHECK-FIXES: {{^}}class CClassWithForwardDecl;
+// CHECK-FIXES: {{^}}class CClassWithForwardDecl {
+class class_with_forward_decl {
+  int __value;
+};
+