[Bug c/99707] New: missing -Woverflow in floating-point to integer conversion for known but non-constant value

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99707

            Bug ID: 99707
           Summary: missing -Woverflow in floating-point to integer
                    conversion for known but non-constant value
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider the following code (from PR93806 Comment 29):

#include <stdio.h>

int main (void)
{
  volatile double d = -1.0;
  double x = d;
  unsigned int i = x;

  printf ("%u\n", i);
  if (x == -1.0)
    printf ("%u\n", i);
  return 0;
}

First note that if instead of "unsigned int i = x;", one has "unsigned int i =
-1.0;", then one gets a warning with GCC 10.2.1:

conv-warn.c: In function ‘main’:
conv-warn.c:7:20: warning: overflow in conversion from ‘double’ to ‘unsigned
in’ changes value from ‘-1.0e+0’ to ‘0’ [-Woverflow]
    7 |   unsigned int i = -1.0;
      |                    ^

But the original code does not give any warning, even though with -O1, GCC uses
the fact that x == -1.0 to optimize and give a strange result (because the
variable i appears to have two different values):

4294967295
0

Thus GCC should know about the undefined behavior and warn.