gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec1.C

// { dg-do run { target c++20 } }

#include <cassert>

template<typename T>
  concept C = __is_class(T);

template<typename T>
  concept D = C<T> && __is_empty(T);

struct X { } x;
struct Y { int n; } y;

template<typename T> void g(T) { } // #1
template<C T> void g(T) { }        // #2
template<D T> void g(T) { }     // #3

int called;

template<> void g(int) { called = 1; } // Specialization of #1
template<> void g<X>(X) { called = 2; } // Specialization of #3
template<> void g(Y) { called = 3; } // Specialization of #2

int main() {
  g(0);
  assert(called == 1);
  g(x);
  assert(called == 2);
  g(y);
  assert(called == 3);
}