// { dg-options "-std=c++17 -fconcepts" }

template<typename T>
  concept bool C() { return __is_class(T); }

template<typename T>
  concept bool D() { return C<T>() and __is_empty(T); }

template<template<typename Q> requires C<Q>() class X>
  struct S { };

// An unconstrained template can be used as an argument for any
// constrained template template parameter.
template<typename A> struct T0 { };
S<T0> x1;

// Matching constraints are valid.
template<typename A> requires C<A>() struct T1 { };
S<T1> x2;

int main() { }