[Bug c++/67720] New: [concepts] bug with recursive constrained function

["ryan.burn at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67720

            Bug ID: 67720
           Summary: [concepts] bug with recursive constrained function
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ryan.burn at gmail dot com
  Target Milestone: ---

The below code is valid but gives this error with gcc:

main.cpp: In instantiation of âauto f(const std::tuple<_El0, _El ...>&)
requires predicate((cpt_BooleanFalseConstant<decltype(p((get<0>)(f::t)))>)())
[with TFirst = std::integral_constant<int, 9>; TRest =
{std::integral_constant<int, 3>, std::integral_constant<int, 5>}]â:
main.cpp:34:11:   required from âauto f(const std::tuple<_El0, _El ...>&)
requires predicate((cpt_BooleanFalseConstant<decltype(p((get<0>)(f::t)))>)())
[with TFirst = std::integral_constant<int, 6>; TRest =
{std::integral_constant<int, 9>, std::integral_constant<int, 3>,
std::integral_constant<int, 5>}]â
main.cpp:44:6:   required from here
main.cpp:34:11: error: call of overloaded
âf(std::tuple<std::integral_constant<int, 3>, std::integral_constant<int, 5>
>)â is ambiguous
   return f(std::tuple<TRest...>());
           ^
main.cpp:24:6: note: candidate: auto f(const std::tuple<_El0, _El ...>&)
requires predicate((cpt_BooleanTrueConstant<decltype(p((get<0>)(f::t)))>)())
[with TFirst = std::integral_constant<int, 3>; TRest =
{std::integral_constant<int, 5>}]
 auto f(const std::tuple<TFirst, TRest...>& t)
      ^
main.cpp:31:6: note: candidate: auto f(const std::tuple<_El0, _El ...>&)
requires predicate((cpt_BooleanFalseConstant<decltype(p((get<0>)(f::t)))>)())
[with TFirst = std::integral_constant<int, 3>; TRest =
{std::integral_constant<int, 5>}]
 auto f(const std::tuple<TFirst, TRest...>& t)

////////////////////////////////////////////////////////////
#include <type_traits>
#include <tuple>

template <class X>
  requires requires {
    X::value;
  }
constexpr bool lift_boolean_constant_value = X::value;

template <class X> concept bool cpt_BooleanTrueConstant() {
  return requires { requires lift_boolean_constant_value<X>; };
}

template <class X> concept bool cpt_BooleanFalseConstant() {
  return requires { requires !lift_boolean_constant_value<X>; };
}

template<int X>
auto p(std::integral_constant<int, X>) {
  return std::integral_constant<bool, (X < 5)>();
}

template<class TFirst, class... TRest>
auto f(const std::tuple<TFirst, TRest...>& t)
  requires cpt_BooleanTrueConstant<decltype(p(std::get<0>(t)))>()
{
  return 1;
}

template<class TFirst, class... TRest>
auto f(const std::tuple<TFirst, TRest...>& t)
  requires cpt_BooleanFalseConstant<decltype(p(std::get<0>(t)))>()
{
  return f(std::tuple<TRest...>());
}

int main() {
  std::tuple<
    std::integral_constant<int, 6>,
    std::integral_constant<int, 9>,
    std::integral_constant<int, 3>,
    std::integral_constant<int, 5>
  > t;
  f(t);
  return 0;
}
////////////////////////////////////////////////////////////