[Bug libstdc++/77395] New: std::is_constructible is false for type constructible via implicit conversion operator affecting std::tuple

[j.s.mueller-roemer at gmx dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77395

            Bug ID: 77395
           Summary: std::is_constructible is false for type constructible
                    via implicit conversion operator affecting std::tuple
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j.s.mueller-roemer at gmx dot net
  Target Milestone: ---

Created attachment 39504
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39504&action=edit
minimal example using std::tuple

The attached code used to compile on G++ 5.4 but no longer does in G++ 6.1 and
6.2, as std::tuple's constructor now checks for std::is_constructible which
erroneously returns false, despite the existence of an implicit conversion
operator.