Type incompatible with itself in GCC 5 when using -Wwrite-strings

Question

Consider:

char f(const char (*x)[4]);

void foo(void) {
    typeof(*"FOO") x[4];
    f(&x);
}

Compile with -Wwrite-strings:

gcc-5 -c gcc5.c  -Wwrite-strings

You get:

gcc5.c: In function ‘foo’:
gcc5.c:5:7: warning: passing argument 1 of ‘f’ from incompatible pointer type [-Wincompatible-pointer-types]
     f(&x);
       ^
gcc5.c:1:6: note: expected ‘const char (*)[4]’ 
   but argument is of type ‘const char (*)[4]’
 char f(const char (*x)[4]);
      ^

Looks like a bug in gcc, unless I'm missing something?

Note: -Wwrite-strings changes the type of literal strings:

When compiling C, give string constants the type "const char[length]"

Answer 1

For me it's indeed a bug in gcc 5.

gcc documentation says

-Wwrite-strings

When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning.

So with this declaration:

typeof(*"FOO") x[4];

then &x is of type const char (*)[4] when -Wwrite-strings is present. In Standard C, &x is of type char (*)[4].

This small function:

void foo(void) {
    typeof(*"FOO") x[4];
    printf("%d\n", __builtin_types_compatible_p(typeof(&x), const char (*)[4]));
    printf("%d\n", __builtin_types_compatible_p(typeof(&x), char (*)[4]));
}

prints:

1
0

with gcc 5.3 and -Wwrite-strings. So we can see that gcc 5.3 correctly identifies &x as of type const char (*)[4] with -Wwrite-strings.

gcc should then accept argument &x when calling a function that has a const char (*)[4] parameter. Warning for incompatible type is IMHO then a bug in gcc.

(This bug probably didn't show in previous versions of gcc simply because gcc failed (another bug) to identify &x as a const char (*)[4] with -Wwrite-strings in previous gcc versions. I tested with gcc 4.9.2 and gcc-6-20151206.)