[Bug libstdc++/99417] New: [C++17] std::variant assignment fails to compile

[Bug libstdc++/99417] New: [C++17] std::variant assignment fails to compile

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99417

            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=(const Original&);
    ~C();
};
void testc(std::variant<C>& vc, const Original& original) {
    vc = original;
}

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


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

[Bug libstdc++/99417] [C++17] std::variant assignment fails to compile

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99417

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED
            Version|unknown                     |11.0
   Target Milestone|---                         |12.3

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think this was a bug, at least not when you reported this. It was the
required behaviour prior to https://wg21.link/lwg3585

In the C++17 and C++20 standards variant<C>::operator=(const Original&) is
specified as:

(13.1) — If *this holds a Tj, assigns std::forward<T>(t) to the value contained
in *this.

(13.2) — Otherwise, if is_nothrow_constructible_v<Tj, T> ||
!is_nothrow_move_constructible_v<Tj> is true, equivalent to
emplace<j>(std::forward<T>(t)).

(13.3) — Otherwise, equivalent to operator=(variant(std::forward<T>(t))).


The second bullet is why the 'noexcept' on the move ctor matters, and it means
that we use the third bullet, but that won't compile because variant<C> is not
move assignable, because C is not move assignable.

So libc++ and MSVC were non-conforming in accepting this.

LWG 3585 changed the third bullet so that move assignment isn't needed. That
was implemented for libstdc++ by r13-5661-g1395415fdc2d60 and
r12-9450-ga5a03ea09ddf09.