gcc/testsuite/g++.dg/concepts/pr65681.C

// { dg-do compile { target c++17_only } }
// { dg-options "-fconcepts" }

template<typename T>
concept bool C = requires (T t) { t.mf(); };

template<typename T>
concept bool CA1 = C<typename T::ca1_type>;

template<typename T>
concept bool CA2 = CA1<T> && requires () { typename T::ca2_type; };

template<typename T>
concept bool CA3 = CA2<T> && requires () { typename T::ca3_type; };

template<typename T>
concept bool CB1 = requires () { typename T::cb1_type; };

template<typename T>
concept bool CB2 = CB1<T> && requires () { typename T::cb2_type; };

template<typename T>
concept bool CB3 = CB2<T> && requires () { typename T::cb3_type; };


struct MC { void mf(); };
static_assert(C<MC>, "");


struct MA1 { using ca1_type = MC; };
struct MA2 : MA1 { using ca2_type = int; };
struct MA3 : MA2 { using ca3_type = int; };
static_assert(CA1<MA1>, "");
static_assert(CA2<MA2>, "");
static_assert(CA3<MA3>, "");

struct MB1 { using cb1_type = int; };
struct MB2 : MB1 { using cb2_type = int; };
struct MB3 : MB2 { using cb3_type = int; };
static_assert(CB1<MB1>, "");
static_assert(CB2<MB2>, "");
static_assert(CB3<MB3>, "");


template<typename T1, typename T2>
struct S;

template<CA1 T1, CB1 T2>
struct S<T1, T2> // Specialization #1
{
  static constexpr int value = 1;
};

template<CA1 T1, CB2 T2>
  requires (!CA2<T1>)
struct S<T1, T2> // Specialization #2
{
  static constexpr int value = 2;
};

template<CA2 T1, CB3 T2>
  requires (!CA3<T1>)
struct S<T1, T2> // Specialization #3
{
  static constexpr int value = 3;
};

static_assert(S<MA1,MB1>::value == 1, "");
static_assert(S<MA1,MB2>::value == 2, "");
static_assert(S<MA1,MB3>::value == 2, "");
static_assert(S<MA2,MB1>::value == 1, "");
static_assert(S<MA2,MB2>::value == 1, "");
static_assert(S<MA2,MB3>::value == 3, "");
static_assert(S<MA3,MB1>::value == 1, "");
static_assert(S<MA3,MB2>::value == 1, "");
static_assert(S<MA3,MB3>::value == 1, "");