This is gcc (GCC) 14.0.1 20240411 (Red Hat 14.0.1-0) on Fedora 40 x86-64. Put this code into the file t.i:

  extern char const *foo () __attribute__ ((returns_nonnull));
  char const *
  foo ()
  {
    return "abc";
  }

and compile it with:

  gcc -S -O2 -Wsuggest-attribute=returns_nonnull t.i

The output is:

  t.i: In function ‘foo’:
  t.i:3:3: warning: function might be candidate for attribute ‘returns_nonnull’ [-Wsuggest-attribute=returns_nonnull]
      3 |   foo ()
        |   ^~~

This diagnostic is obviously wrong, as foo is already declared with the suggested attribute.