[Clang] prevent setting default lexical access specifier for missing primary declarations

clang/docs/ReleaseNotes.rst

@@ -589,6 +589,7 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure in range calculations for conditional throw expressions. (#GH111854)
 - Clang now correctly ignores previous partial specializations of member templates explicitly specialized for
   an implicitly instantiated class template specialization. (#GH51051)
+- Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -17955,6 +17955,8 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
             << Name;
         Invalid = true;
       }
+      if (TUK == TagUseKind::Declaration)
+        Invalid = true;
     } else if (!PrevDecl) {
       Diag(Loc, diag::warn_decl_in_param_list) << Context.getTagDeclType(New);
     }

clang/test/SemaCXX/enum.cpp

@@ -143,3 +143,11 @@ struct PR28903 {
     })
   };
 };
+
+namespace GH112208 {
+class C {
+  enum E { e = 0 };
+  void f(int, enum E;); // expected-error {{ISO C++ forbids forward references to 'enum' types}} \
+                        // expected-error {{unexpected ';' before ')'}}
+};
+}