[Bug libstdc++/92570] New: clang fails to instantiate std::optional if A is not const copy constructible

[romain.geissler at amadeus dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92570

            Bug ID: 92570
           Summary: clang fails to instantiate std::optional<A> if A is
                    not const copy constructible
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: romain.geissler at amadeus dot com
  Target Milestone: ---

Hi,

The following snippet fails to build with clang >= 8 using libstdc++ >= 8.1.0:

#include <optional>

class A
{
    public:
        A() = default;
        A(A&) = default;
};

void f()
{
    std::optional<A> a1 [[maybe_unused]];
    //std::optional<A> a2(a1);
}

compiled with -std=gnu++17.

gcc is ok with that if we keep commented the last line of "f". However clang
seems to eagerly check if " = default" declaration actually make sense, even if
it's never used. It reads:

In file included from <source>:1:
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/optional:148:7:
error: the parameter for this explicitly-defaulted copy constructor is const,
but a member or base requires it to be non-const
      _Optional_payload_base(const _Optional_payload_base&) = default;
      ^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/optional:354:7:
note: in instantiation of template class 'std::_Optional_payload_base<A>'
requested here
    : _Optional_payload_base<_Tp>
      ^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/optional:511:30:
note: in instantiation of template class 'std::_Optional_payload<A, true,
false, false>' requested here
      _Optional_payload<_Tp> _M_payload;
                             ^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/optional:657:15:
note: in instantiation of template class 'std::_Optional_base<A, false, false>'
requested here
    : private _Optional_base<_Tp>,
              ^
<source>:12:22: note: in instantiation of template class 'std::optional<A>'
requested here
    std::optional<A> a1 [[maybe_unused]];
                     ^
1 error generated.
Compiler returned: 1



Here I am not sure who is really right. Shall libstdc++ extensively use things
like enable_if to conditionally define all these explicitly defaulted functions
to please clang, or is clang too eager in checking this even when it's not
actually used/needed ? I am fine opening a clang bug report if you consider the
issue is on clang side.

Cheers,
Romain