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From: "jonas.rahlf.basf at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101795] New: (x > QNaNf) is not a constant expression
Date: Thu, 05 Aug 2021 22:08:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101795

            Bug ID: 101795
           Summary: (x > QNaNf) is not a constant expression
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jonas.rahlf.basf at gmail dot com
  Target Milestone: ---

Somehow (x > QNaNf) is not a constant expression, according to g++. I could=
 not
find anything in the c++ standard that would support this behaviour.

Both clang and MSVC compile (x > QNaNf) as a constant expression.
Interestingly, (x =3D=3D QNaNf) seems to work fine with g++.


Example:
#include <limits>
constexpr bool canCompile =3D std::numeric_limits<float>::quiet_NaN() > 1;

Compiler output: error: '(+QNaNf > 1.0e+0f)' is not a constant expression
See also: https://godbolt.org/z/vW651jPYh

No extra compile options needed.=20
I noticed this when attempting to compile std::lerp(a,b,NaN) (which is supp=
osed
to be constexpr) as a constant expression.=