1 // Testcase from P1814R0
2 // { dg-do compile { target c++2a } }
3 
4 template <class T> struct identity { using type = T; };
5 template <class T> using identity_t = typename identity<T>::type;
6 template <class T> concept Int = __is_same_as (T, int);
7 
8 template <class T, class U> struct C {
9   C(T, U);			// #1 { dg-message "constraint" }
10 };
11 template<class T, class U>
12 C(T, U) -> C<T, identity_t<U>>; // #2 { dg-message "constraint" }
13 
14 template<class V>
15 using A = C<V *, V *>;
16 
17 template<Int W>
18 using B = A<W>;
19 
20 int i{};
21 double d{};
22 A a1(&i, &i); // { dg-bogus "" "Deduces A<int>" }
23 A a2(i, i);   // { dg-error "" "cannot deduce V * from i" }
24 A a3(&i, &d); // { dg-error "" } #1: Cannot deduce (V*, V*) from (int *, double *)
25                               // #2: Cannot deduce A<V> from C<int *, double *>
26 B b1(&i, &i); // { dg-bogus "" "Deduces B<int>" }
27 B b2(&d, &d); // { dg-error "" "cannot deduce B<W> from C<double *, double *>" }
28