[Bug c/82528] New: Warning for conversion from bool to enum

["fw at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82528

            Bug ID: 82528
           Summary: Warning for conversion from bool to enum
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

I think this program should emit a warning in C mode:

#define TRUE  ((_Bool) 1)
#define FALSE ((_Bool) 0)

typedef enum { a, b, c } result;

result
f (int flag)
{
  if (flag)
    return TRUE;
  else
    return FALSE;
}

Likewise, this should warn as well:

#include <stdbool.h>

typedef enum { a, b, c } result;

result
f (int flag)
{
  if (flag)
    return true;
  else
    return false;
}

This may require the definition of __true and __false as compiler built-ins,
similar to what exists for NULL in the C++ front end, so that the compiler can
keep up the fiction that true and false are the integer constants 1 and 0
(apparently, false is a valid NULL pointer constant, but warning about false
where NULL could be used seems reasonable to me).