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

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

[thakis at chromium dot 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?

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

[thakis at chromium dot org]

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

--- Comment #2 from thakis at chromium dot org ---
That only works on declarations, not in an expression, right?

(We do have a workaround that works with gcc. But "pragma message" matches the
semantics of what we want to do, and it works with MSVC and clang, so it'd be
nice if it worked in gcc too.)

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

[desrt at desrt dot ca]

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

Ryan Lortie <desrt at desrt dot ca> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |desrt at desrt dot ca

--- Comment #3 from Ryan Lortie <desrt at desrt dot ca> ---
We want to do similar things in GLib in order to warn in the middle of
arbitrary expressions or statements expanded from macros (essentially: support
for deprecated macros).