Missing __VA_ARGS__ extension in Microsoft mode

[llvmbot]

Bugzilla Link	12845
Resolution	FIXED
Resolved on	May 17, 2018 09:16
Version	trunk
OS	Windows NT
Blocks	llvm/llvm-bugzilla-archive#13707
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@efriedma-quic,@tritao,@nico

Extended Description

When passing a single argument to a macro which defines a function with a variable arguments list as its' second argument, Clang will incorrectly preprocess such code, leaving the unnecessary comma after the first argument, thus making the resulting code uncompilable. Consider the example:

bool MsgAlert(bool yes_no, int Style, const char* format, ...)
{return true;}
#define PanicAlert(format, ...) MsgAlert(false, 1, format, VA_ARGS)

void func()
{PanicAlert("text");}

Clang preprocessor (the -E flag, in MS compatibility mode) will generate the following (note the excessive comma after "text"):

void func()
{MsgAlert(false, 1, "text", );}

In comparison, the MSVC preprocessor (cl.exe) will generate the correct code:

void func()
{MsgAlert(false, 1, "text" );}

[efriedma-quic]

gcc has a very similar extension... but not quite the same. :(

[llvmbot]

Note that MS will allow empty argument list and remove the trailing comma even when compiling with /Za (disable language extensions).

[llvmbot]

Also, the trailing comma is removed, even if the VA_ARGS is followed by the pasting operator:

#define FOO(x, ...) int a((x, __VA_ARGS__ ## 1))
FOO(1); // error: missing ')'

[llvmbot]

Patch: avakar/clang@bug_12845

[tritao]

Can you send your patch to cfe-commits?

[llvmbot]

Can you send your patch to cfe-commits?

Sure, I'll write tests for it and send it.

[nico]

This seems to be working now.

[tritao]

mentioned in issue llvm/llvm-bugzilla-archive#13707