[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
      |                ^