[Bug c/45780] Warning for arithmetic operations involving C99 _Bool variable

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45780

--- Comment #7 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Uroš Bizjak from comment #0)
> > >> It can be done ultimately, but as a prerequisite, we should have
> > >> warnings in -Wextra for all of
> > >>
> > >> ? boolvar++; ++boolvar;
> > >> ? boolvar--; --boolvar;
> > >> ? boolvar = nonbool;
> > >> ? boolvar & nonbool; boolvar &= nonbool;
> > >> ? boolvar | nonbool; boolvar |= nonbool;
> > >> ? boolvar ^ nonbool; boolvar ^= nonbool;
> > >
> > > Fair enough. I have CCed Manuel, perhaps he is interested in this warning.
> > 
> > I am not sure it fits in -Wconversion. -Wbool-arith perhaps?
> 

I made a testcase that includes all of those:

$ cat 45780.c
#include <stdbool.h>
#include <stddef.h>

static bool f1(int i)
{
  return i;
}

static bool f2(int i)
{
  i = !!i;
  return i;
}

int main(int argc, char **argv)
{
  bool a = true;
  bool b = a++;
  bool c = ++b;
  bool d = argc;
  bool e = a & argc;
  bool f = b | argc;
  bool g = c ^ argc;
  f &= argc;
  g |= argc;
  e ^= argc;
  if (f1(argc))
    e--;
  else
    --e;
  if (!!argc)
    return ((argv != NULL) ? d : ((f > g) ? e : (f << g)));
  else
    return f2(argc);
}
$

gcc's -Wbool-operation currently catches the increments and decrements, but
none of the rest of the operations:

$ /usr/local/bin/gcc -c -Wall -Wextra -Wbool-compare -Wint-in-bool-context
-Wparentheses -pedantic -Wconversion 45780.c
45780.c: In function 'main':
45780.c:18:13: warning: increment of a boolean expression [-Wbool-operation]
   18 |   bool b = a++;
      |             ^~
45780.c:19:12: warning: increment of a boolean expression [-Wbool-operation]
   19 |   bool c = ++b;
      |            ^~
45780.c:28:6: warning: decrement of a boolean expression [-Wbool-operation]
   28 |     e--;
      |      ^~
45780.c:30:5: warning: decrement of a boolean expression [-Wbool-operation]
   30 |     --e;
      |     ^~
$