Add support for #pragma system_header with -fms-extensions

[hans]

Summary

Clang already supports the pragma prefixed by "GCC" or "clang".

MSVC has more recently added support for the pragma, but without any prefix; see https://devblogs.microsoft.com/cppblog/broken-warnings-theory/#external-headers

[hans]

clang/test/Lexer/pragma-operators.cpp

22

I had to change the pragma here, because now we will warn about system_header being ignored in a non-header file which would break this test.

clang/test/Preprocessor/pragma_sysheader.c

0–1

Adding -std=c99 is necessary, because in newer language modes (which are now the default), Clang doesn't warn about repeated typedef declarations, so the test wasn't actually testing the system_header pragma anymore.

5–7

Moving the .h file to Inputs/ is just a cleanup.

[aaron.ballman]

LGTM, though you could add a test that #pragma system_header gives a diagnostic outside of -fms-extensions mode if you think that's valuable.

[hans]

In D104770#2835596, @aaron.ballman wrote:

LGTM, though you could add a test that #pragma system_header gives a diagnostic outside of -fms-extensions mode if you think that's valuable.

Thanks! Yes, I'll do that, and I'll also add a test to see that it actually warns when no system_header is used.

clang/lib/Lex/Pragma.cpp

void Preprocessor::RegisterBuiltinPragmas() {
   AddPragmaHandler(new PragmaRegionHandler("endregion"));
 
   // MS extensions.
   if (LangOpts.MicrosoftExt) {
     AddPragmaHandler(new PragmaWarningHandler());
     AddPragmaHandler(new PragmaExecCharsetHandler());
     AddPragmaHandler(new PragmaIncludeAliasHandler());
     AddPragmaHandler(new PragmaHdrstopHandler());
+    AddPragmaHandler(new PragmaSystemHeaderHandler());
   }
 
   // Pragmas added by plugins
   for (const PragmaHandlerRegistry::entry &handler :
        PragmaHandlerRegistry::entries()) {
     AddPragmaHandler(handler.instantiate().release());
   }
 }

clang/test/Lexer/pragma-operators.cpp

#define A(X) extern "C" { __pragma(warning(push)) \
   int X() { return 0; } \
 }
 #define B(X) A(X)
 #pragma warning(push)
 B(foo)
 #pragma warning(pop)
 
 #define pragma_L _Pragma(L"GCC diagnostic push")
-#define pragma_u8 _Pragma(u8"system_header")
+#define pragma_u8 _Pragma(u8"pack(1)")
 #define pragma_u _Pragma(u"GCC diagnostic pop")
 #define pragma_U _Pragma(U"comment(lib, \"libfoo\")")
 #define pragma_R _Pragma(R"(clang diagnostic ignored "-Wunused")")
 #define pragma_UR _Pragma(UR"(clang diagnostic error "-Wunused")")
 #define pragma_hello _Pragma(u8R"x(message R"y("Hello", world!)y")x")
 // CHECK: int n =
 // CHECK: #pragma GCC diagnostic push
-// CHECK: #pragma system_header
+// CHECK: #pragma pack(1)
 // CHECK: #pragma GCC diagnostic pop
 // CHECK: #pragma comment(lib, "libfoo")
 // CHECK: #pragma clang diagnostic ignored "-Wunused"
 // CHECK: #pragma clang diagnostic error "-Wunused"
 // CHECK: #pragma message("\042Hello\042, world!")
 // CHECK: 0;
 int n = pragma_L pragma_u8 pragma_u pragma_U pragma_R pragma_UR pragma_hello 0;
 

clang/test/Preprocessor/Inputs/pragma_sysheader.h

+#if defined(CLANG)
+#pragma clang system_header
+// expected-no-diagnostics
+#elif defined(GCC)
+#pragma GCC system_header
+// expected-no-diagnostics
+#elif defined(MS)
+#pragma system_header
+// expected-no-diagnostics
+#else
+// expected-warning@+1{{unknown pragma ignored}}
+#pragma system_header
+
+// expected-note@+4{{previous definition is here}}
+// expected-warning@+4{{redefinition of typedef 'x' is a C11 feature}}
+#endif
+
+typedef int x;
+typedef int x;

clang/test/Preprocessor/pragma_sysheader.h

-#pragma GCC system_header
-typedef int x;
-typedef int x;
-

clang/test/Preprocessor/pragma_sysheader.c

-// RUN: %clang_cc1 -verify -pedantic %s -fsyntax-only
-// RUN: %clang_cc1 -E %s | FileCheck %s
-// expected-no-diagnostics
-// rdar://6899937
-#include "pragma_sysheader.h"
+// RUN: %clang_cc1 -verify -std=c99 -Wunknown-pragmas -pedantic %s -fsyntax-only
+// RUN: %clang_cc1 -verify -std=c99 -Wunknown-pragmas -pedantic %s -fsyntax-only -DGCC
+// RUN: %clang_cc1 -verify -std=c99 -Wunknown-pragmas -pedantic %s -fsyntax-only -DCLANG
+// RUN: %clang_cc1 -verify -std=c99 -Wunknown-pragmas -pedantic %s -fsyntax-only -fms-extensions -DMS
 
+// rdar://6899937
+#include "Inputs/pragma_sysheader.h"
 
+// RUN: %clang_cc1 -E %s | FileCheck %s
 // PR9861: Verify that line markers are not messed up in -E mode.
 // CHECK: # 1 "{{.*}}pragma_sysheader.h" 1
-// CHECK-NEXT: # 2 "{{.*}}pragma_sysheader.h" 3
-// CHECK-NEXT: typedef int x;
-// CHECK-NEXT: typedef int x;
-// CHECK-NEXT: # 6 "{{.*}}pragma_sysheader.c" 2
+// CHECK-NEXT: # 12 "{{.*}}pragma_sysheader.h"
+// CHECK: typedef int x;
+// CHECK: typedef int x;
+// CHECK-NEXT: # 8 "{{.*}}pragma_sysheader.c" 2