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From: "ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96675] New: tautological-compare warning emitted for NTTP
 bitwise comparison
Date: Tue, 18 Aug 2020 10:50:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96675

            Bug ID: 96675
           Summary: tautological-compare warning emitted for NTTP bitwise
                    comparison
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

template<char c>
constexpr bool f(char d) {
    return 'a' <=3D c && c <=3D 'z' ? (d | 0x20) =3D=3D c :
        'A' <=3D c && c <=3D 'Z' ? (d & ~0x20) =3D=3D c :
        d =3D=3D c;
}
static_assert(f<'p'>('P'));

In instantiation of 'constexpr bool f(char) [with char c =3D 'p']':
<source>:7:21:   required from here
<source>:4:44: warning: bitwise comparison always evaluates to false
[-Wtautological-compare]
    4 |         'A' <=3D c && c <=3D 'Z' ? (d & ~0x20) =3D=3D c :
      |                                ~~~~~~~~~~~~^~~~

Per godbolt, this happened somewhere between 10.1.0 and 10.2.0.

Motivation is
https://github.com/capnproto/capnproto/commit/c38629e5a4b8ce9561b81cb23ea5f=
c39cbd93eb7=