[Bug c/106988] New: subscripting a string literal is not an integer constant expression but __builtin_strlen is

[Bug c/106988] New: subscripting a string literal is not an integer constant expression but __builtin_strlen is

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106988

            Bug ID: 106988
           Summary: subscripting a string literal is not an integer
                    constant expression but __builtin_strlen is
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In C mode, GCC rejects a subscript expression whose operand is a string literal
in contexts where an integer constant expression is required but accepts an
equivalent expression involving __builtin_strlen.  Although conforming, it's
inconsistent and unnecessarily restrictive.  Accepting both (either with
-Wpedantic) would also be conforming and more useful to programmers who need it
but don't think of using the built-in workaround.

$ cat a.c && gcc -S -Wall -Wpedantic a.c
#define EMPTY1(S) (__builtin_strlen(S) == 0)
_Static_assert(!EMPTY1("1"), "");    // ok

#define EMPTY2(S) (S[0] == 0)
_Static_assert(!EMPTY2("1"), "");    // error
a.c:5:16: error: expression in static assertion is not constant
    5 | _Static_assert(!EMPTY2("1"), "");    // error
      |                ^

[Bug c/106988] subscripting a string literal is not an integer constant expression but __builtin_strlen is

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106988

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=44094

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is an exact dup of bug 89408 and basically the same as PR 44094.

*** This bug has been marked as a duplicate of bug 89408 ***

[Bug c/106988] subscripting a string literal is not an integer constant expression but __builtin_strlen is

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106988

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|DUPLICATE                   |---
             Status|RESOLVED                    |UNCONFIRMED
           Keywords|diagnostic                  |rejects-valid

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
After experimenting with this some more I think the decision to reject this and
the other similar requests should be reconsidered.  GCC accepts many equivalent
nonconstant expressions in contexts where they are required (see below). 
Rejecting the simplest of them seems arbitrary and is (as is evident from the
duplicate requests) unhelpful to C programmers and surprising those used to the
C++ behavior.

$ cat a.c && gcc -S -Wall -Wpedantic a.c
#include <string.h>

_Static_assert (memcmp ("", "", 1) == 0, "");  // okay in C mode only
_Static_assert (memchr ("", 0, 1) != 0, "");   // okay in C mode only
_Static_assert (strlen ("") == 0, "");         // okay in C and C++
_Static_assert (strcmp ("", "") == 0, "");     // okay in C and C++
_Static_assert ("" == "", "");                 // okay in C and C++

_Static_assert (*"" == 0, "");                 // error in C only

a.c:3:36: warning: expression in static assertion is not an integer constant
expression [-Wpedantic]
    3 | _Static_assert (memcmp ("", "", 1) == 0, "");  // okay in C mode only
      |                 ~~~~~~~~~~~~~~~~~~~^~~~
a.c:4:35: warning: expression in static assertion is not an integer constant
expression [-Wpedantic]
    4 | _Static_assert (memchr ("", 0, 1) != 0, "");   // okay in C mode only
      |                 ~~~~~~~~~~~~~~~~~~^~~~
a.c:5:29: warning: expression in static assertion is not an integer constant
expression [-Wpedantic]
    5 | _Static_assert (strlen ("") == 0, "");         // okay in C and C++
      |                 ~~~~~~~~~~~~^~~~
a.c:6:33: warning: expression in static assertion is not an integer constant
expression [-Wpedantic]
    6 | _Static_assert (strcmp ("", "") == 0, "");     // okay in C and C++
      |                 ~~~~~~~~~~~~~~~~^~~~
a.c:7:20: warning: comparison with string literal results in unspecified
behavior [-Waddress]
    7 | _Static_assert ("" == "", "");                 // okay in C and C++
      |                    ^~
a.c:7:20: warning: expression in static assertion is not an integer constant
expression [-Wpedantic]
    7 | _Static_assert ("" == "", "");                 // okay in C and C++
      |                 ~~~^~~~~
a.c:9:21: error: expression in static assertion is not constant
    9 | _Static_assert (*"" == 0, "");                 // error in C only
      |                 ~~~~^~~~