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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/103511] New: __builtin_bit_cast requires a constructor call
Date: Wed, 01 Dec 2021 03:46:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103511

            Bug ID: 103511
           Summary: __builtin_bit_cast requires a constructor call
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

According to [class.prop] p1, a trivially copyable class may have all of its
constructors deleted. And according to [bit.cast] p1, an object of such a c=
lass
type can be created or copied by std::bit_cast since C++20.
But currently gcc's __builtin_bit_cast seemly requires that the target type=
 has
at least one eligible copy or move constructor.

#include <bit>

class Weird {
    Weird(const Weird&) =3D delete;
    Weird(Weird&&) =3D delete;
    // works when either constructor is changed to =3D default
    Weird &operator=3D(const Weird&) =3D default;
    Weird &operator=3D(Weird&&) =3D default;

    int value_;
};

int main()
{
    auto x [[maybe_unused]] =3D std::bit_cast<Weird>(0);
}

wandbox links:
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https://wandbox.org/permlink/lrMmtVw1mt5nZjqY=