[Clang] Deleting an incomplete enum type is not an error (#119077)

The changes introduced in #97733 accidentally prevented to delete an
incomplete enum
(the validity of which has been confirmed by CWG2925
    
Fixes #99278

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -799,6 +799,7 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure caused by mangled names with invalid identifiers. (#GH112205)
 - Fixed an incorrect lambda scope of generic lambdas that caused Clang to crash when computing potential lambda
   captures at the end of a full expression. (#GH115931)
+- Clang no longer rejects deleting a pointer of incomplete enumeration type. (#GH99278)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExprCXX.cpp

@@ -3747,7 +3747,8 @@ Sema::ActOnCXXDelete(SourceLocation StartLoc, bool UseGlobal,
     } else if (!Pointee->isDependentType()) {
       // FIXME: This can result in errors if the definition was imported from a
       // module but is hidden.
-      if (!RequireCompleteType(StartLoc, Pointee,
+      if (Pointee->isEnumeralType() ||
+          !RequireCompleteType(StartLoc, Pointee,
                                LangOpts.CPlusPlus26
                                    ? diag::err_delete_incomplete
                                    : diag::warn_delete_incomplete,

clang/test/SemaCXX/new-delete.cpp

@@ -540,6 +540,22 @@ namespace PR10504 {
   void f(A *x) { delete x; } // expected-warning {{delete called on 'PR10504::A' that is abstract but has non-virtual destructor}}
 }
 
+#if __cplusplus >= 201103L
+enum GH99278_1 {
+    zero = decltype(delete static_cast<GH99278_1*>(nullptr), 0){}
+    // expected-warning@-1 {{expression with side effects has no effect in an unevaluated context}}
+};
+template <typename = void>
+struct GH99278_2 {
+  union b {};
+  struct c {
+    c() { delete d; }
+    b *d;
+  } f;
+};
+GH99278_2<void> e;
+#endif
+
 struct PlacementArg {};
 inline void *operator new[](size_t, const PlacementArg &) throw () {
   return 0;