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From: "Darrell.Wright at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102916] New: cmath constexpr can lead to ODR
 violations/incorrect results
Date: Sun, 24 Oct 2021 17:27:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102916

            Bug ID: 102916
           Summary: cmath constexpr can lead to ODR violations/incorrect
                    results
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Darrell.Wright at gmail dot com
  Target Milestone: ---

Because many of the cmath functions are constexpr, even in conformance mode=
 of
the compiler, this can lead to ODR issues.  It is detectable in C++20 and g=
ives
incorrect results.

#include <cmath>

template<auto F>
consteval auto foo( ) {
  if constexpr (requires {
                  []<int>() {}.template operator()<(F(), void(), 0)>();
                }) {
                        return 0;
  } else {=20
                return 1;
        }
}


auto a =3D foo<[]{ return std::sqrt( 4.0 ); }>( );

https://gcc.godbolt.org/z/M6zGTbTfM


gcc returns 0, clang/libstdc++ and MSVC return 1.

I believe that in conformance -std=3Dc++20 -pedantic mode that gcc should n=
ot
mark these methods as constexpr=