[Clang][Sema] Check the return value of DiagnoseClassNameShadow in ActOnEnumConstant (llvm#143754)

Static analysis flagged that we were not checking the return value of
DiagnoseClassNameShadow when we did so everywhere else. Modifying this
case to match how other places uses it makes sense and does not change
behavior. Likely if this check fails later actions will fail as well but
it is more correct to exit early.

Author: shafik

clang/lib/Sema/SemaDecl.cpp

@@ -20270,9 +20270,10 @@ Decl *Sema::ActOnEnumConstant(Scope *S, Decl *theEnumDecl, Decl *lastEnumConst,
   // different from T:
   // - every enumerator of every member of class T that is an unscoped
   // enumerated type
-  if (getLangOpts().CPlusPlus && !TheEnumDecl->isScoped())
-    DiagnoseClassNameShadow(TheEnumDecl->getDeclContext(),
-                            DeclarationNameInfo(Id, IdLoc));
+  if (getLangOpts().CPlusPlus && !TheEnumDecl->isScoped() &&
+      DiagnoseClassNameShadow(TheEnumDecl->getDeclContext(),
+                              DeclarationNameInfo(Id, IdLoc)))
+    return nullptr;
 
   EnumConstantDecl *New =
     CheckEnumConstant(TheEnumDecl, LastEnumConst, IdLoc, Id, Val);

clang/test/CXX/class/class.mem/p13.cpp

@@ -114,3 +114,12 @@ template<typename B> struct CtorDtorName : B {
   CtorDtorName();
   ~CtorDtorName(); // expected-error {{identifier 'CtorDtorName' after '~' in destructor name does not name a type}}
 };
+
+struct S { // expected-note {{'S' declared here}}
+  enum E {
+    R = 11,
+    S = 12  // expected-error {{member 'S' has the same name as its class}}
+  };
+  static_assert(E::R == 11, "E::R is not 11");
+  static_assert(E::S == 12, "E::S is not 12");  // expected-error {{no member named 'S' in 'S::E'}}
+};