Suppress non-conforming GNU paste extension in all standard-conforming modes

The GNU token paste extension that removes the comma in , ## __VA_ARGS__
conflicts with C99/C++11's requirements when a variadic macro has no
named parameters: according to the standard, an invocation as FOO()
gives it a single empty argument, and concatenation of anything with an
empty argument is well-defined. For this reason, the GNU extension was
already disabled in C99 standard-conforming mode. It was not yet
disabled in C++11 standard-conforming mode.

The associated comment suggested that GCC keeps this extension enabled
in C90/C++03 standard-conforming mode, but it actually does not, so
rather than adding a check for C++ language version, this change simply
removes the check for C language version.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D91913

Author: hvdijk

clang/lib/Lex/TokenLexer.cpp

@@ -148,12 +148,11 @@ bool TokenLexer::MaybeRemoveCommaBeforeVaArgs(
     return false;
 
   // GCC removes the comma in the expansion of " ... , ## __VA_ARGS__ " if
-  // __VA_ARGS__ is empty, but not in strict C99 mode where there are no
-  // named arguments, where it remains.  In all other modes, including C99
-  // with GNU extensions, it is removed regardless of named arguments.
+  // __VA_ARGS__ is empty, but not in strict mode where there are no
+  // named arguments, where it remains.  With GNU extensions, it is removed
+  // regardless of named arguments.
   // Microsoft also appears to support this extension, unofficially.
-  if (PP.getLangOpts().C99 && !PP.getLangOpts().GNUMode
-        && Macro->getNumParams() < 2)
+  if (!PP.getLangOpts().GNUMode && Macro->getNumParams() < 2)
     return false;
 
   // Is a comma available to be removed?

clang/test/Preprocessor/macro_fn_comma_swallow2.c

@@ -1,9 +1,12 @@
 // Test the __VA_ARGS__ comma swallowing extensions of various compiler dialects.
 
 // RUN: %clang_cc1 -E %s | FileCheck -check-prefix=GCC -strict-whitespace %s
+// RUN: %clang_cc1 -E -std=c90 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=c99 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=c11 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -x c++ %s | FileCheck -check-prefix=GCC -strict-whitespace %s
+// RUN: %clang_cc1 -E -x c++ -std=c++03 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
+// RUN: %clang_cc1 -E -x c++ -std=c++11 %s | FileCheck -check-prefix=C99 -strict-whitespace %s
 // RUN: %clang_cc1 -E -std=gnu99 %s | FileCheck -check-prefix=GCC -strict-whitespace %s
 // RUN: %clang_cc1 -E -fms-compatibility %s | FileCheck -check-prefix=MS -strict-whitespace %s
 // RUN: %clang_cc1 -E -DNAMED %s | FileCheck -check-prefix=GCC -strict-whitespace %s