[MS] Fix assert handling enum forward decls in hasVisibleDefinition

An enum may be considered to be a complete type if it was forward
declared. It may be declared with a fixed underlying type, or, in MSVC
compatiblity mode, with no type at all.

Previously, the code was written with special handling for fixed enums.
I generalized the code to check if the underlying integer type is known,
which should be the case when targetting the MSVC C++ ABI.

Fixes PR45409

Author: rnk

clang/lib/Sema/SemaType.cpp

@@ -8060,10 +8060,10 @@ bool Sema::hasVisibleDefinition(NamedDecl *D, NamedDecl **Suggested,
   } else if (auto *ED = dyn_cast<EnumDecl>(D)) {
     if (auto *Pattern = ED->getTemplateInstantiationPattern())
       ED = Pattern;
-    if (OnlyNeedComplete && ED->isFixed()) {
-      // If the enum has a fixed underlying type, and we're only looking for a
-      // complete type (not a definition), any visible declaration of it will
-      // do.
+    if (OnlyNeedComplete && !ED->getIntegerType().isNull()) {
+      // If the enum has an integer type, it may have been forward declared.
+      // Since we're only looking for a complete type (not a definition), any
+      // visible declaration of it will do.
       *Suggested = nullptr;
       for (auto *Redecl : ED->redecls()) {
         if (isVisible(Redecl))

clang/test/Modules/Inputs/ms-enums/A.h

@@ -0,0 +1 @@
+enum fwd_enum;

clang/test/Modules/Inputs/ms-enums/B.h

@@ -0,0 +1 @@
+#include "A.h"

clang/test/Modules/Inputs/ms-enums/module.map

@@ -0,0 +1,2 @@
+module A { header "A.h" }
+module B { header "B.h" }

clang/test/Modules/ms-enums.cpp

@@ -0,0 +1,12 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fms-extensions -fms-compatibility -x c++ -std=c++20 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs/ms-enums %s -verify -fno-modules-error-recovery
+
+#include "B.h"
+// expected-note@A.h:1 {{previous declaration is here}}
+// expected-note@A.h:1 2 {{previous definition is here}}
+
+fwd_enum gv_enum; // expected-error {{must be imported}}
+
+struct Foo {
+  enum fwd_enum enum_field; // expected-error 2 {{must be imported}}
+};