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

template<typename T>
  concept bool C1 = __is_same_as(T, int);

template<int N>
  concept bool C2 = N == 0;

template<template<typename> class X>
  concept bool C3 = true;

template<typename> struct Foo;

template<C1... Ts = int> struct S1; // { dg-error "default argument" }
template<C1... = int> struct S2; // { dg-error "default argument" }
template<C2... Ns = 0> struct S3; // { dg-error "default argument" }
template<C2... = 0> struct S4; // { dg-error "default argument" }
template<C3... Ts = Foo> struct S5; // { dg-error "default argument" }
template<C3... = Foo> struct S6; // { dg-error "default argument" }