Say I was trying to implement a concept meowable that

1. Integral types are meowable.
2. _{Class types with member function meow are meowable. This is in the final target but the current question doesn't focus on it.}
3. Tuple-like types with only meowable elements are meowable.
4. _{std::ranges::range with meowable elements are meowable. This is in the final target but the current question doesn't focus on it.}

Then I came up with this implementation(simplified as I could):

#include <concepts>
#include <type_traits>
#include <ranges>
#include <utility>
#include <tuple>

template<class T>
concept meowable_builtin = std::integral<T>;

template<class T, std::size_t I>
concept has_tuple_element = requires (T t) {
  typename std::tuple_element<I, T>::type;
  { get<I>(t) } -> std::convertible_to<std::tuple_element_t<I, T>&>;
};

template<class T>
concept tuple_like = requires {
    typename std::tuple_size<T>::type;
    { std::tuple_size_v<T> } -> std::convertible_to<std::size_t>;
  } &&
  []<std::size_t...I>(std::index_sequence<I...>) {
    return (has_tuple_element<T, I> && ...);
  } (std::make_index_sequence<std::tuple_size_v<T>>{});

template<class               T> struct is_meowable:    std::false_type{};
template<meowable_builtin    T> struct is_meowable<T>: std::true_type{};

template<tuple_like T>
struct is_meowable<T>
  : std::bool_constant<
      []<std::size_t...I>(std::index_sequence<I...>) {
        return (is_meowable<std::tuple_element_t<I, T>>::value && ...);
      } (std::make_index_sequence<std::tuple_size_v<T>>{})
    > {};

template<class T>
concept meowable_tuple = tuple_like<T> && is_meowable<T>::value;

template<class T>
concept meowable = is_meowable<T>::value;

static_assert(meowable<int>);
//static_assert(tuple_like<std::tuple<int>>);
static_assert(is_meowable<std::tuple<int>>::value);

But some compilers don't like it (https://godbolt.org/z/5vMTEhTdq):

1. GCC-12 and above:   internal compiler error.
2. GCC-11:             accepted.
3. Clang-13 and above: static_assert fired.
4. MSVC-v19:           accepted.

However, if I uncomment the second last line of code, all compilers are happy. (Instantiation point of concepts?)

So my questions are:

1. Why this behavior? (compiler bug or something like "ill-formed NDR"?)
2. How can I achieve my target?

CodePudding user response：

1. Why this behavior? (compiler bug or something like "ill-formed NDR"?)

This is apparently a bug of GCC-trunk and Clang-trunk, the issue here is that GCC/Clang doesn't properly handle the template partial specialization based on the concept initialized by the lambda. Reduced

template<class>
concept C = [] { return true; } ();

template<class T> 
struct S {};

template<class T>
  requires C<T>
struct S<T> { constexpr static bool value = true; };

// static_assert(C<int>);
static_assert(S<int>::value);

2. How can I achieve my target?

Replace lambda with the template function based on the reduced result

template<class T, std::size_t...I>
constexpr bool all_has_tuple_element(std::index_sequence<I...>) {
  return (has_tuple_element<T, I> && ...);
}

template<class T>
concept tuple_like = requires {
  typename std::tuple_size<T>::type;
  { std::tuple_size_v<T> } -> std::convertible_to<std::size_t>;
} && all_has_tuple_element<T>(std::make_index_sequence<std::tuple_size_v<T>>{});

Demo

CodePudding user response：

Here is a simple recursive SFINAE solution for C 17 (and 11 for some adjustments):
https://godbolt.org/z/7sMjfq7ze

#include <type_traits>

template <typename T, typename = void>
struct has_meow : std::false_type {};

// this is a customization point to deduce tuple-like traits
// you can define some default logic here,
// but for the sake of example let it be false by default
template <typename>
struct is_tuple_like : std::false_type {};

template <typename T>
struct has_meow<T, std::void_t<decltype(std::declval<T>().mew())>> : std::true_type {};

template <typename T>
struct meowable : std::disjunction<std::is_integral<T>, has_meow<T>> {};

template <template <typename...> class TupleT, typename ... T>
struct meowable<TupleT<T...>> : std::conjunction<
                                          is_tuple_like<TupleT<T...>>,
                                          std::disjunction<meowable<T>...
                                       > {};

#include <tuple>
#include <array>

// this will also catch std::pair
template <typename ... T>
struct is_tuple_like<std::tuple<T...>> : std::true_type {};

template <typename T, size_t N>
struct is_tuple_like<std::array<T, N>> : std::true_type {};

struct cat
{
    void mew() {}
};

int main()
{
    static_assert(meowable<int>::value);
    static_assert(meowable<std::tuple<int, long>>::value);
    static_assert(meowable<cat>::value);
    static_assert(meowable<std::tuple<cat, long>>::value);
    static_assert(meowable<std::tuple<int, std::tuple<cat, long>>>::value);

    return 0;
};