[clang][Sema]fix crash of invalid friend declaration with storage-class specifier (#190597)

Fix an assertion failure in Sema::ActOnFriendTypeDecl when parsing an
invalid friend type declaration that incorrectly includes a
storage-class specifier (e.g., 'static', 'extern', 'register').

Root cause:
If the type specifier is marked as invalid, DeclSpec::Finish returns
early. However, even when the type specifier is invalid, some other
checks can still be performed instead of skipping everything.

This change allows necessary checks to proceed, preventing the
assertion in ActOnFriendTypeDecl and enabling proper error diagnostics.

Fixes: #186569

Co-authored-by: victorl <liuvicsen@gmail.com>

Author: firstmoonlight

clang/docs/ReleaseNotes.rst

@@ -733,6 +733,7 @@ Bug Fixes to C++ Support
 - Fixed a use-after-free bug when parsing default arguments containing lambdas in declarations with template-id declarators. (#GH196725)
 - Fixed a crash in constant evaluation using placement new on an array which was later initialized. (#GH196450)
 - Fixed an issue where Clang incorrectly accepted invalid unqualified uses of local nested class names outside their declaring scope. (#GH184622)
+- Fixed a crash when parsing invalid friend declaration with storage-class specifier. (#GH186569)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/DeclSpec.h

@@ -889,6 +889,10 @@ class DeclSpec {
   /// DeclSpec is guaranteed self-consistent, even if an error occurred.
   void Finish(Sema &S, const PrintingPolicy &Policy);
 
+  void CheckTypeSpec(Sema &S, const PrintingPolicy &Policy);
+
+  void CheckFriendSpec(Sema &S, const PrintingPolicy &Policy);
+
   const WrittenBuiltinSpecs& getWrittenBuiltinSpecs() const {
     return writtenBS;
   }

clang/lib/Sema/DeclSpec.cpp

@@ -1162,6 +1162,20 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
 
   // Check the type specifier components first. No checking for an invalid
   // type.
+  CheckTypeSpec(S, Policy);
+
+  CheckFriendSpec(S, Policy);
+
+  assert(!TypeSpecOwned || isDeclRep((TST)TypeSpecType));
+
+  // Okay, now we can infer the real type.
+
+  // TODO: return "auto function" and other bad things based on the real type.
+
+  // 'data definition has no type or storage class'?
+}
+
+void DeclSpec::CheckTypeSpec(Sema &S, const PrintingPolicy &Policy) {
   if (TypeSpecType == TST_error)
     return;
 
@@ -1441,6 +1455,9 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
     S.Diag(ConstexprLoc, diag::warn_cxx20_compat_consteval);
   else if (getConstexprSpecifier() == ConstexprSpecKind::Constinit)
     S.Diag(ConstexprLoc, diag::warn_cxx20_compat_constinit);
+}
+
+void DeclSpec::CheckFriendSpec(Sema &S, const PrintingPolicy &Policy) {
   // C++ [class.friend]p6:
   //   No storage-class-specifier shall appear in the decl-specifier-seq
   //   of a friend declaration.
@@ -1498,14 +1515,6 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
     FS_explicit_specifier = ExplicitSpecifier();
     FS_virtualLoc = FS_explicitLoc = SourceLocation();
   }
-
-  assert(!TypeSpecOwned || isDeclRep((TST) TypeSpecType));
-
-  // Okay, now we can infer the real type.
-
-  // TODO: return "auto function" and other bad things based on the real type.
-
-  // 'data definition has no type or storage class'?
 }
 
 bool DeclSpec::isMissingDeclaratorOk() {

clang/test/CXX/class/class.friend/p6.cpp

@@ -19,4 +19,5 @@ class A {
 #else
   friend thread_local class G; // expected-error {{'thread_local' is invalid in friend declarations}}
 #endif
+  friend register enum; // expected-error {{expected identifier or '{'}} expected-error {{'register' is invalid in friend declarations}}
 };