On Friday, 31 January 2020 at 00:51:45 UTC, Ben Jones wrote: > writeln(typeof(f3).stringof); > writeln(isInstanceOf!(Nullable, f3)); In the first one, you check typeof, but not in the second one. writeln(isInstanceOf!(Nullable, typeof(f3))); // true The template argument there was T above - the type, not the value.