1 // { dg-do compile { target c++17 } } 2 3 template <class T> struct A { 4 A(T); // #1 5 A(const A&); // #2 6 }; 7 8 template <class T> A(T) -> A<T>; // #3 9 10 A a (42); // uses #3 to deduce A<int> and initializes with #1 11 A b = a; // uses #2 (not #3) to deduce A<int> and initializes with #2; #2 is more specialized 12 13 template <class T> A(A<T>) -> A<A<T>>; // #4 14 15 A b2 = a; // uses #4 to deduce A<A<int>> and initializes with #1; #4 is as specialized as #2 16 17 template <class,class> struct same; 18 template <class T> struct same<T,T> {}; 19 20 same<decltype(a),A<int>> s1; 21 same<decltype(b),A<int>> s2; 22 same<decltype(b2),A<A<int>>> s3; 23