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From: "eyalroz at technion dot ac.il" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95148] New: -Wtype-limits always-false warning triggered
 despite comparison being avoided
Date: Fri, 15 May 2020 09:25:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95148

            Bug ID: 95148
           Summary: -Wtype-limits always-false warning triggered despite
                    comparison being avoided
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz at technion dot ac.il
  Target Milestone: ---

Consider the following program:

  #include <type_traits>

  int main() {
          unsigned x { 5 };
          return (std::is_signed<decltype(x)>::value and (x < 0)) ? 1 : 0;
  }

when compiling it with GCC versions 11.0 20200511, 10.1, 9.2.1, 8.3.0, I get
the warning:

  a.cpp:5:52: warning: comparison of unsigned expression < 0 is always false
[-Wtype-limits]

I should not be getting this warning, because when x is unsigned, the
comparison is never performed, due to the short-circuit semantics of `and`.
This can be easily determined by the compiler - and probably is. No less
importantly, the author of such a line in a program clearly specified his/h=
er
intent here with this check.=20

clang doesn't seem to issue a warn inf does come to pass.=