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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99957] New: Ill-formed std::pair construction supported
Date: Wed, 07 Apr 2021 12:44:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99957

            Bug ID: 99957
           Summary: Ill-formed std::pair construction supported
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: minor
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This shouldn't compile:

#include <utility>

struct move_only
{
    move_only() =3D default;
    move_only(move_only&&) =3D default;
};

move_only mo() { return {}; }

std::pair<void*, move_only> p0(0, mo());
std::pair<void*, move_only> p1({}, mo());

According to the standard, these constructors are considered:

  pair(const first_type&, const second_type&);
  template<class U1, class U2> pair(U1&&, U2&&);

For both p0 and p1, the first does not participate in overload resolution
because second_type is not copy constructible.

For p0, the second constructor does not participate because U1 is deduced as
int, and a pointer cannot be constructed from int. For p1, U1 cannot be ded=
uced
from {}.

The code compiles with libstdc++ because we have additional non-standard
constructors, added by PR 40925 and described in
https://gcc.gnu.org/pipermail/libstdc++/2021-April/052299.html (where I
proposed to deprecate these non-standard constructors, and eventually remove
them).=