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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99417] New: [C++17] std::variant assignment fails to
 compile
Date: Fri, 05 Mar 2021 17:12:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99417

            Bug ID: 99417
           Summary: [C++17] std::variant assignment fails to compile
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/b3P6Ta
#include <variant>
struct Original {};
struct C {
    C(const Original&);
    C(C&&) noexcept;
    C& operator=3D(const Original&);
    ~C();
};
void testc(std::variant<C>& vc, const Original& original) {
    vc =3D original;
}

In file included from <source>:1:/include/c++/11.0.1/variant:1445:3:
error: call to deleted member function 'operator=3D'
                operator=3D(variant(std::forward<_Tp>(__rhs)));
                ^~~~~~~~~
<source>:13:8: note: in instantiation of function template specialization
'std::variant<C>::operator=3D<const Original &>' requested here
    vc =3D original;
       ^
/include/c++/11.0.1/variant:1368:16: note: candidate function has been
implicitly deleted
      variant& operator=3D(const variant&) =3D default;
               ^
/include/c++/11.0.1/variant:1431:2: note: candidate template ignored:
requirement '__not_self<std::variant<C> &&>' was not satisfied [with _Tp =3D
std::variant<C>]
        operator=3D(_Tp&& __rhs)
        ^
1 error generated.


Removing the keyword `noexcept` from the move-constructor makes this compil=
e.
It compiles fine (with or without `noexcept`) on libc++ and MSVC STL.=