Treat enumerator_too_large as an extension in MS ABI mode

[DaveBartolomeo]

Summary

When compiling as C targeting the MS ABI, but with -fno-ms-compatibility, an enumerator initializer that is not representable as an int is treated as an error. This is correct according to the C standard, but Clang already treats it as an extension both in -fms-compatibility mode and when compiling as C for non-MS ABI targets. It seemed odd that it would be treated as a hard error only in this one particular configuration, so I relaxed it to be an extension when targeting the MS ABI, regardless of MS compatibility mode.

There are dozens of occurrences of this issue in Windows SDK headers. By treating it as an extension, Clang users can include <windows.h> in a C program without having to use -fms-compatibility.

[majnemer]

This requires an associated test case.

lib/Sema/SemaDecl.cpp

14354–14355

I would remove the getLangOpts().MSVCCompat.

[rnk]

I think this is just a consequence of https://llvm.org/bugs/show_bug.cgi?id=27098

We should just fix the underlying bug so that EnumDecl::isFixed() doesn't return true for plain enums in C or C++ when targeting Windows.

[DaveBartolomeo]

I'll attempt to fix the underlying issue https://llvm.org/bugs/show_bug.cgi?id=27098 instead.

lib/Sema/SemaDecl.cpp

else {
         if (Enum->isFixed()) {
           EltTy = Enum->getIntegerType();
 
           // In Obj-C and Microsoft mode, require the enumeration value to be
           // representable in the underlying type of the enumeration. In C++11,
           // we perform a non-narrowing conversion as part of converted constant
           // expression checking.
           if (!isRepresentableIntegerValue(Context, EnumVal, EltTy)) {
-            if (getLangOpts().MSVCCompat) {
+            if (getLangOpts().MSVCCompat ||
+                Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+              // Treat as an extension in MSVC compat mode or if using the
+              // MSVC ABI. We already treat the equivalent case in C as an
+              // extension if we're not in Microsoft mode. Several Windows
+              // headers define enums with initializers like 0x80000000 and
+              // 0xffffffff.
               Diag(IdLoc, diag::ext_enumerator_too_large) << EltTy;
               Val = ImpCastExprToType(Val, EltTy, CK_IntegralCast).get();
             } else
               Diag(IdLoc, diag::err_enumerator_too_large) << EltTy;
           } else
             Val = ImpCastExprToType(Val, EltTy,
                                     EltTy->isBooleanType() ?
                                     CK_IntegralToBoolean : CK_IntegralCast)