[Bug c++/66841] New: [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[Bug c++/66841] New: [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66841

            Bug ID: 66841
           Summary: [concepts] bogus error "invalid reference to function
                    concept" when function concept is overloaded
           Product: gcc
           Version: c++-concepts
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

This is related to the discussion in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66834. Because of the kind
mismatch error described in 66834, the straightforward implementation of
Constructible doesn't work, and the concept must be overloaded. But once it's
overloaded, uses of it cause of "invalid reference to function concept" error.


template <class T, class U>
concept bool Same =
  __is_same_as(T, U);

template <class T, class U>
concept bool ExplicitlyConvertible() { return
  Same<T, U> ||
  requires(T&& t) {
    static_cast<U>((T&&)t);
  };
}

template <class T>
concept bool Constructible() { return
  requires {
    T{ };
  };
}

template <class T, class U>
concept bool Constructible() { return
  ExplicitlyConvertible<U, T>() ||
  requires (U&& u) {
    T{ (U&&)u };
  };
}

template <class T, class U, class V, class...Args>
concept bool Constructible() { return
  requires (U&& u, V&& v, Args&&...args) {
    T{ (U&&)u, (V&&)v, (Args&&)args... };
  };
}

template <class, class...>
constexpr bool core_constructible() { return false; }

template <class T, class...Args>
  requires Constructible<T, Args...>() // ERROR HERE
constexpr bool core_constructible() { return false; }


Yields:

../cmcstl2/scratch/constructible.cpp:40:23: error: invalid reference to
function concept ‘template<class T, class U, class V, class ... Args> constexpr
bool Constructible()’
 requires Constructible<T, Args...>()
                       ^
>From gcc-bugs-return-492046-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Sat Jul 11 18:18:27 2015
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From: "paolo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/58734] [C++11] Template template parameter pack instantiation problem
Date: Sat, 11 Jul 2015 18:18:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idX734

--- Comment #4 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Sat Jul 11 18:17:41 2015
New Revision: 225713

URL: https://gcc.gnu.org/viewcvs?rev"5713&root=gcc&view=rev
Log:
2015-07-11  Paolo Carlini  <paolo.carlini@oracle.com>

        PR c++/58734
        * g++.dg/cpp0x/variadic-ttp2.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/variadic-ttp2.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/66841] [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[andrew.n.sutton at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66841

--- Comment #1 from Andrew Sutton <andrew.n.sutton at gmail dot com> ---
The program is ill-formed. In this line:

   requires Constructible<T, Args...>() // ERROR HERE

There's no single declaration of Constructible that can be matched to those
template arguments. You would need one with this signature: <typename T,
typename... Args>.

I think defining Constructible so that it backends into a type trait will give
you the behavior you're looking for.

template<typename T, typename... Args>
concept bool Constructible() {
  return std::is_constructible<T, Args...>::value;
}

[Bug c++/66841] [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66841

--- Comment #2 from Eric Niebler <eric.niebler at gmail dot com> ---
This answer is deeply unsatisfying. I want valid expressions, not traits. And
if std::is_constructible doesn't do *exactly* what I want (and it doesn't) I
have to author my own trait, when what I want to do is author my own concept.

I'm not trying to do anything exotic here. There should be a way to do it
without resorting to metaprogramming trickery and SFINAE.

[Bug c++/66841] [concepts] bogus error "invalid reference to function concept" when function concept is overloaded

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66841

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Let's keep the variadic concepts discussion in 66834.

*** This bug has been marked as a duplicate of bug 66834 ***