[Bug c++/67348] New: [concepts] Constraints, special member functions, and default/delete

["Casey at Carter dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67348

            Bug ID: 67348
           Summary: [concepts] Constraints, special member functions, and
                    default/delete
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

Having recently implemented N4542's variant, it would be very convenient to be
able to write e.g.:

template <class...Ts>
  requires (is_destructible<Ts>::value && ...)
struct variant {
  ~variant() { /* ... */ }
  ~variant()
    requires (is_trivially_destructible<Ts>::value && ...) = default;

  variant(variant&&) { /* ... */ }
  variant(variant&&)
    requires (is_trivially_move_constructible<Ts>::value && ...) = default;

  variant& operator=(variant&&) { /* ... */ }
  variant& operator=(variant&&)
    requires (is_trivially_move_assignable<Ts>::value && ...) = default;

  // ...similar treatment for copy construction / assignment...
};

static_assert(std::is_trivially_destructible<variant<int, float>>());
static_assert(!std::is_trivially_destructible<variant<int, vector<int>>>());

static_assert(std::is_trivially_move_constructible<variant<int, float>>());
static_assert(!std::is_trivially_move_constructible<variant<int,
vector<int>>>());

Formally, constraints are forbidden for destructors and non-template
constructors since the Concepts TS does not change the required syntax forms in
[class.ctor]/1 and [class.dtor]/1 to allow for an optional requires-clause;
this is likely an oversight. GCC currently allows a requires-clause on
constructor and destructor declarations, but breaks badly when multiple
destructors are overloaded or when constraints are used in conjunction with
default/delete.

I suppose this is therefore both a bug report - compiler fails to diagnose as
ill-formed code which truly should be well-formed ;) - and a feature request
for both the compiler and the Concepts TS. I would be happy to provide
specification wording for constraints on special member functions and semantic
interaction with default/delete if I could interest anyone in implementing the
behavior in the compiler as an extension.