[Bug c++/82171] New: Cant use std::declval in concept testing map operator[]

[rwdougla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82171

            Bug ID: 82171
           Summary: Cant use std::declval in concept testing map
                    operator[]
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rwdougla at gmail dot com
  Target Milestone: ---

static_assert fires inside c++/7.2.0/type_traits:2259:7

Godbolt link: https://godbolt.org/g/raan9j

Pasted here: (compiler flags are -std=c++1z -fconcepts)
#include <utility>
#include <type_traits>
#include <iterator>

template<typename T>
concept bool MapLike = requires(T t) {
    {t[std::declval<typename T::value_type::first_type>()]}
        -> typename T::value_type::second_type;
};

void test(MapLike T) {}

#include <map>
#include <vector>

int main() {
    test(std::map<int, int>{});
    //test(std::vector<std::pair<int, int>>{});
}


Workaround requires bankshot through nested requirement:
https://godbolt.org/g/j7kuKF

#include <utility>
#include <type_traits>
#include <iterator>

template<typename MapT, typename ValueT>
concept bool HasIndexOp = requires (MapT t, ValueT v) {
    { t[v] } -> typename MapT::value_type::second_type;
};

template<typename T>
concept bool MapLike = requires(T t) {
    requires HasIndexOp<T, typename T::value_type::first_type>;
};

void test(MapLike T) {}

#include <map>
#include <vector>

int main() {
    test(std::map<int, int>{});
    //test(std::vector<std::pair<int, int>>{});
}