[Bug c/83656] New: missing -Wbuiltin-declaration-mismatch on declaration without prototype

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83656

            Bug ID: 83656
           Summary: missing -Wbuiltin-declaration-mismatch on declaration
                    without prototype
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ issues a -Wbuiltin-declaration-mismatch for the memcpy declaration in the
test case below but GCC does not.  Since -Wstrict-prototypes is disabled by
default even in pedantic mode (see bug 82922) this makes it easy to overlook
such declarations in source code and miss invalid calls to them as in the
following test case:

$ cat d.c && gcc -O2 -S -Wall -Wextra -Wpedantic d.c
#if __cplusplus
extern "C" void* memcpy (...);
#else
void* memcpy ();
#endif

void f (char *d)
{
  memcpy (3, d, "123");
}