[Bug c/126757] New: please diagnose invalid 'const' attribute on functions

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126757

            Bug ID: 126757
           Summary: please diagnose invalid 'const' attribute on functions
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bruno at clisp dot org
  Target Milestone: ---

As GCC makes more and more optimizations, 'const' attributes on
functions that are in fact not const (as defined in
https://gcc.gnu.org/onlinedocs/gcc/Common-Attributes.html)
can lead to undefined behaviour. See e.g.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78463
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119041
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125916
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/42361
https://gitlab.gnome.org/GNOME/glib/-/work_items/3984

It would therefore be useful to diagnose common cases of such
invalid 'const' attributes. (I'm aware that it cannot diagnose
all such cases, because that would be equivalent to solving the
halting problem.)

How to reproduce:
1. Save this file as foo.c.
================================================================
double phi (void) __attribute__ ((__const__));

double phi (void)
{
  static int counter;
  return (++counter) * 1.618033988749894848;
}

#include <stdio.h>
int main ()
{
  for (int repeat = 10; repeat > 0; repeat--)
    printf ("%g\n", phi ());
}
================================================================
2.
$ gcc -Wall -O2 foo.c
<no warning>