[Bug libstdc++/91456] std::function and std::is_invocable_r do not understand guaranteed elision

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91456

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2019-08-15
            Version|9.1.1                       |10.0
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's a related problem in std::is_invocable_r:

// { dg-options "-std=gnu++17" }
// { dg-do compile { c++17 } }
#include <type_traits>

struct Immovable {
  Immovable() = default;
  Immovable(const Immovable&) = delete;
  Immovable& operator=(const Immovable&) = delete;
};

static_assert(std::is_invocable_r_v<Immovable, Immovable(*)()>);
static_assert(std::is_invocable_r_v<const Immovable, Immovable(*)()>);
static_assert(std::is_invocable_r_v<Immovable, const Immovable(*)()>);

The assertions fail because is_invocable_r uses is_convertible, which doesn't
know about guaranteed elision.