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From: "johelegp at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/107378] New: `get_if` implementation "is not a
 constant expression"
Date: Mon, 24 Oct 2022 15:36:00 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: johelegp at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107378

            Bug ID: 107378
           Summary: `get_if` implementation "is not a constant expression"
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
                CC: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/cf1T7q6Tj.

```C++
#include <variant>
static_assert([v =3D std::variant<int*>{}] { return get_if<0>(&v); }());
```

```
<source>:2:67: error: non-constant condition for static assertion
    2 | static_assert([v =3D std::variant<int*>{}] { return get_if<0>(&v); =
}());
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
In file included from <source>:1:
<source>:2:67:   in 'constexpr' expansion of '<lambda closure
object><lambda()>{std::variant<int*>{std::__detail::__variant::_Variant_bas=
e<int*>{std::__detail::__variant::_Move_assign_base<true,
int*>{std::__detail::__variant::_Copy_assign_base<true,
int*>{std::__detail::__variant::_Move_ctor_base<true,
int*>{std::__detail::__variant::_Copy_ctor_base<true,
int*>{std::__detail::__variant::_Variant_storage<true,
int*>{std::__detail::__variant::_Variadic_union<int*>{std::__detail::__vari=
ant::_Uninitialized<int*,
true>{0}}, 0}}}}}}}}.<lambda()>()'
<source>:2:60:   in 'constexpr' expansion of 'std::get_if<0, int*>((& v))'
/opt/compiler-explorer/gcc-trunk-20221024/include/c++/13.0.0/variant:1183:1=
1:
error: '((&<anonymous>.<lambda()>::__v) !=3D 0)' is not a constant expressi=
on
 1183 |       if (__ptr && __ptr->index() =3D=3D _Np)
      |           ^~~~~
Compiler returned: 1
```=