[Bug c/77292] value range propagation (VRP) would improve -Wlogical-not-parentheses

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77292

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to M Welinder from comment #3)
> The actual code I got this warning from was...
> 
> 		if (!lower_tail == (p > phalf)) {
> 
> where lower_tail is an int and p and phalf are doubles.

Well, that changes things because I agree that in the above case the
warning is bogus.


> That's simply a comparison of two booleans.  Note, that the hinted-at code
> 
>    !(lower_tail == (p > phalf))
> 
> is dubious: comparing an int to a booleans.
> 
> > this isn't visible to the compiler when it analyzes
> >   return !a == b;
> 
> I am not aware of a rule that requires the compiler to ignore context
> when considering warnings.  It certainly does consider context when
> it issues "might be used uninitialized" warnings, so why not here?

Because it's not done as in the frontend there is no data-flow available.

So, testcase:

int
foo (int a, int b)
{
  return !a == (a < b);
}

t.c: In function ‘foo’:
t.c:4:13: warning: logical not is only applied to the left hand side of
comparison [-Wlogical-not-parentheses]
   return !a == (a < b);
             ^~