[clang] Don't emit warn_cxx_ms_struct when MSBitfields is enabled globally

[mstorsjo]

Summary

This diagnostic (which defaults to an error, added in 95833f33bda6c92e746e0b0007b69c2c30bfc693) was (as far as I can see) intended to clearly point out cases where the C++ ABI won't match the Microsoft C++ ABI, for cases when this is enabled via a pragma over a region of code.

The MSVC compatible struct layout feature can also be enabled via a compiler option (-mms-bitfields). If enabled that way, one essentially can't compile any C++ code unless also building with -Wno-incompatible-ms-struct (which GCC doesn't support, and projects developed with GCC aren't setting).

For the MinGW target, it's expected that the C++ ABI won't match the MSVC one, if this option is used for getting the struct layout to match MSVC.

[mstorsjo]

Fixed the formatting

[amccarth]

Thanks. This seems well-contained and the test changes look good.

(rnk is on leave and unlikely to respond.)

clang/lib/Sema/SemaDeclCXX.cpp

void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
   // whether this class uses any C++ features that are implemented
   // completely differently in MSVC, and if so, emit a diagnostic.
   // That diagnostic defaults to an error, but we allow projects to
   // map it down to a warning (or ignore it).  It's a fairly common
   // practice among users of the ms_struct pragma to mass-annotate
   // headers, sweeping up a bunch of types that the project doesn't
   // really rely on MSVC-compatible layout for.  We must therefore
   // support "ms_struct except for C++ stuff" as a secondary ABI.
-  if (Record->isMsStruct(Context) &&
+  // Don't emit this diagnostic if the feature was enabled as a
+  // language option (as opposed to via a pragma or attribute), as
+  // the option -mms-bitfields otherwise essentially makes it impossible
+  // to build C++ code, unless this diagnostic is turned off.
+  if (Record->isMsStruct(Context) && !Context.getLangOpts().MSBitfields &&
       (Record->isPolymorphic() || Record->getNumBases())) {
     Diag(Record->getLocation(), diag::warn_cxx_ms_struct);
   }
 
   checkClassLevelDLLAttribute(Record);
   checkClassLevelCodeSegAttribute(Record);
 
   bool ClangABICompat4 =

clang/test/SemaCXX/ms_struct.cpp

-// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
-// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
 // RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_ERROR -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DNO_PRAGMA -mms-bitfields -verify -triple armv7-apple-darwin9 -std=c++11 %s
 
+#ifndef NO_PRAGMA
 #pragma ms_struct on
+#endif
 
 struct A {
   unsigned long a:4;
   unsigned char b;
 };
 
 struct B : public A {
 #ifdef TEST_FOR_ERROR
   // expected-error@-2 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
-#else
+#elif defined(TEST_FOR_WARNING)
   // expected-warning@-4 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
 #endif
   unsigned long c:16;
   int d;
   B();
 };
 
 static_assert(__builtin_offsetof(B, d) == 12,
   "We can't allocate the bitfield into the padding under ms_struct");
 
 // rdar://16178895
 struct C {
 #ifdef TEST_FOR_ERROR
   // expected-error@-2 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
-#else
+#elif defined(TEST_FOR_WARNING)
   // expected-warning@-4 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
 #endif
   virtual void foo();
   long long n;
 };
 
 static_assert(__builtin_offsetof(C, n) == 8,
               "long long field in ms_struct should be 8-byte aligned");
+#if !defined(TEST_FOR_ERROR) && !defined(TEST_FOR_WARNING)
+// expected-no-diagnostics
+#endif