Bug 14797 – template with inout parameter `foo(String: inout(CharT)[], CharT)` should be implicitly instanciated as foo!(char[], char)

In the following code, foo(String: inout(CharT)[], CharT) should be implicitly instanciated as foo!(char[], char) but it does not. I checked it doesn't work with dmd trunk v2.068-devel-4bc3d3d on Mac OSX. ---- inout(dchar)[] foo(String: inout(CharT)[], CharT)(String str) { return typeof(return).init; } void main() { char[] str = "str".dup; auto b = foo!(inout(char)[], char)(str); // it works auto a = foo(str); // it doesn't work but it should } ---- Output: error.d(10): Error: template error.foo cannot deduce function from argument types !()(char[]), candidates are: error.d(1): error.foo(String : inout(CharT)[], CharT)(String str) P.S. Its severity may be `enhancement` instead of normal.

This is expected behavior. What you have is a specialization for inout(CharT) where str is inout(CharT)[], and you are passing in just char[], which doesn't match the specialization. IFTI breaks down in certain ways with specializations, but this should work if you have an inout(char)[] to pass in. I think all you need to do to fix your example is: inout(dchar)[] foo(CharT)(inout(CharT)[] str)