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From: "jan at twosigma dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105344] New: std::source_location::curent() seemingly
 treated as a pure function in template specializations
Date: Fri, 22 Apr 2022 10:11:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105344

            Bug ID: 105344
           Summary: std::source_location::curent() seemingly treated as a
                    pure function in template specializations
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jan at twosigma dot com
  Target Milestone: ---

When using std::source_location::current().line() in certain contexts on two
different lines, gcc seems to incorrectly think the expression refers to the
same value.

Testcase:

#include <source_location>
#include <utility>

template <int i, class =3D void> struct foo;

// Two following two specializations are different, yet gcc errors out,
// claiming they are the same.
template <int i>
struct foo<i, std::enable_if_t<i =3D=3D std::source_location::current().lin=
e()>> {
    static constexpr int num =3D i;
};

template <int i>
struct foo<i, std::enable_if_t<i =3D=3D std::source_location::current().lin=
e()>> {
    static constexpr int num =3D i;
};

This outputs:

<source>:14:8: error: redefinition of 'struct foo<i, typename std::enable_i=
f<(i
=3D=3D std::source_location::current().std::source_location::line()), void>=
::type>'
   14 | struct foo<i, std::enable_if_t<i =3D=3D
std::source_location::current().line()>> {
      |=20=20=20=20=20=20=20
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:8: note: previous definition of 'struct foo<i, typename
std::enable_if<(i =3D=3D
std::source_location::current().std::source_location::line()), void>::type>'
    9 | struct foo<i, std::enable_if_t<i =3D=3D
std::source_location::current().line()>> {
      |=20=20=20=20=20=20=20
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 1

https://godbolt.org/z/ozf1MbG3n shows this code works fine under MSVC.=