[Bug c/77876] New: -Wbool-operation rejects useful code involving '~'

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77876

            Bug ID: 77876
           Summary: -Wbool-operation rejects useful code involving '~'
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at gnu dot org
  Target Milestone: ---

GCC's recently-added -Wbool-operation flag rejects useful code like this:

#include <time.h>
enum { BILLION = 1000 * 1000 * 1000 };
time_t foo (time_t s, int res) { return s & ~ (res == 2 * BILLION); }

This is a simplified version of Gnulib code using '~' that runs afoul of
-Wbool-operation; see the bug report here:

http://lists.gnu.org/archive/html/bug-gnulib/2016-10/txtbxk_mHAW_p.txt

Rather than contort user code to pacify this misguided warning, I suggest
making the warning more useful. The fundamental bug here is not applying ~ to a
boolean; it's storing ~x into a boolean. More generally, the problem occurs
when converting an expression that GCC can't prove to be 0 or 1 to bool. GCC
should check for that instead. This would catch not only thinkos with ~ and ++
and --, but also similar thinkos involving other integer and floating-point
operations. And it would correctly accept the Gnulib code.