[Bug preprocessor/79948] _Pragma(“GCC error”) is processed differently inside and outside of a #if directive

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79948

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-03-08
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to fail|                            |4.1.3, 4.2.2, 4.3.2, 4.6.0,
                   |                            |5.4.0, 6.3.0, 7.0

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
I'll confirm this as a bug with the test case below which is expected to
compile without an error (and does with other compilers I tested, including
Clang 5.0, EDG eccp 4.13, IBM XLC 13.1.3, Oracle cc 5.13, and Microsoft Visual
C/C++).

It seems to me that in 'return Y (E)' GCC first expands the E macro into the
_Pragma, then executes the _Pragma, and then expands the Y() macro.

I think it should expand all the macros first, i.e., first E, then Y, then X,
and then execute any _Pragmas.

$ cat t.c && gcc -S t.c
#define E  _Pragma ("GCC error \"This is an error\"")

#define X(a) my##a
#define Y(a) X(a)

int my_Pragma (const char*);

int f (void)
{
  return Y (E);
};
t.c: In function ‘f’:
t.c:10:11: error: This is an error
   return Y (E);
           ^~~~~             
t.c:3:14: error: ‘my’ undeclared (first use in this function)
 #define X(a) my##a
              ^
t.c:4:14: note: in expansion of macro ‘X’
 #define Y(a) X(a)
              ^
t.c:10:10: note: in expansion of macro ‘Y’
   return Y (E);
          ^
t.c:3:14: note: each undeclared identifier is reported only once for each
function it appears in
 #define X(a) my##a
              ^
t.c:4:14: note: in expansion of macro ‘X’
 #define Y(a) X(a)
              ^
t.c:10:10: note: in expansion of macro ‘Y’
   return Y (E);
          ^