Bug 23798 – Type inference should traverse trivial aliases

See also: https://forum.dlang.org/thread/[email protected] In C++, you can infer instantiation through simple alias templates: ``` template <typename T, int M, int N> struct Matrix { }; template <typename T> using Vector3 = Matrix<T, 3, 1>; template<typename T> void baz(Vector3<T> vector) {} int main() { baz(Vector3<int>()); } ``` There is nothing special about the parameter being `Vector3<int>`; this also works: ``` baz(Matrix<int, 3, 1>()); ``` C++ tries to match `Vector3<T>` with `Matrix<T, int, int>`, notices that `Vector3<T>` is a trivial alias to `Matrix<T, 3, 1>`, and matches with that instead. If you try to do the analogous thing in D: ``` struct Matrix(T, int M, int N) {} alias Vector3(T) = Matrix!(T, 3, 1); void foo(T = Vector3!U, U)(T arg) { } void main() { foo!(Vector3!int, int)(Vector3!int()); foo(Vector3!int()); } ``` The first call works, but the second fails to infer that `U` can be `int`. D here should notice that `Vector3(T)` is a trivial alias to another template instance and do a simple parameter substitution, treating `foo` as equivalent to `void foo(T = Matrix!(U, 3, 1), U)(T arg)`.

Oops: Dupe of https://issues.dlang.org/show_bug.cgi?id=1807 , thanks jmh530 *** This issue has been marked as a duplicate of issue 1807 ***