Viewing file:  concepts-template-parm6.C (1.26 KB) -rw-r--r--
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// { dg-do compile { target c++20 } }
template<typename... Ts> struct are_same;
template<>
struct are_same<> {
static constexpr bool value = true;
};
template<typename T>
struct are_same<T> {
static constexpr bool value = true;
};
template<typename T, typename U, typename... Ts>
struct are_same<T, U, Ts...> {
static constexpr bool value =
__is_same_as(T, U) && are_same<U, Ts...>::value;
};
template<typename... Ts>
concept Same = are_same<Ts...>::value;
template<typename T>
concept Int = __is_same_as(T, int);
// NOTE: The use of Same is misleading; it constraints each T, not
// the pack of Ts.
template<Same... Ts> struct S1 { }; // requires (C<Ts> && ...)
S1<int, int, int> s1;
S1<int, int, bool> s2;
// // NOTE: The use of Same is misleading; it constraints each deduced
// // type, not the pack of Ts.
template<Same auto... Xs> struct S2 { }; // requires (C<X> && ...) with each X deduced
S2<true, true, true> s3;
S2<true, true, 'a'> s4; // OK
template<Int... Ts> struct S3 { }; // requires (C<Ts> && ...)
S3<int, int, char> x0; // { dg-error "template constraint failure" }
template<Int auto... Xs> struct S4 { }; // requires (C<X> && ...) with each X deduced
S4<0, 1, 2, 'a'> x1; // { dg-error "placeholder constraints not satisfied" }
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