[Bug preprocessor/60875] New: `_Pragma("message \"foo\")"` doesn't work in expression contexts.

["thakis at chromium dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60875

            Bug ID: 60875
           Summary: `_Pragma("message \"foo\")"` doesn't work in
                    expression contexts.
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thakis at chromium dot org

Actual:

$ cat test.cc
int main() {
  return _Pragma("message \"hello\"") 1234;
}
$ ~/gcc482prefix/bin/gcc -c test.cc  -Wall
test.cc: In function ‘int main()’:
test.cc:2:1: error: ‘#pragma’ is not allowed here
   return _Pragma("message \"my warning\"") 1234;
 ^
test.cc:2:44: error: expected ‘;’ before numeric constant
   return _Pragma("message \"my warning\"") 1234;
                                            ^
test.cc:2:49: warning: statement has no effect [-Wunused-value]
   return _Pragma("message \"my warning\"") 1234;
                                                ^

Expected (which is what clang does, and what MSVS does with `
__pragma(message...`):

Display the message instead.

(clang's output:

$ ../../../chrome/src/third_party/llvm-build/Release+Asserts/
bin/clang -c test.cc -Wall
test.cc:2:10: warning: my warning [-W#pragma-messages]
  return _Pragma("message \"my warning\"") 1234;
         ^
<scratch space>:2:2: note: expanded from here
 message "my warning"
 ^
1 warning generated.
)


A silly workaround is to use _Pragma("message_workaround...), then gcc will
warn  like so:

test.cc:2:10: warning: unknown pragma ignored [-Wunknown-pragmas]
  return _Pragma("message_workaround \"my warning\"") 1234;
         ^


It'd be nice if _Pragma(message...) could be used directly, instead of having
to rely on -Wunknown-pragmas for this functionality.



(The usecase we have for this in Chromium is that we want to make the compiler
print a diagnostic every time some symbols are used, so we define the symbols
as

  #define MY_SYMBOL _Pragma("message \"foo\"") SYMBOL

This works fine in clang and MSVC, but for gcc we have to use a dummy pragma
and rely on -Wunknown-pragmas.)
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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/60875] `_Pragma("message \"foo\")"` doesn't work in expression contexts.
Date: Thu, 17 Apr 2014 20:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`875

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Use deprecated attribute instead?