Why this template code does not compile?

[Paul Backus]

On Wednesday, 13 March 2019 at 17:38:02 UTC, Victor Porton wrote:
> Why this template code does not compile?
>
> ///
> module runnable;
>
> import std.meta;
> import std.typecons;
>
> template FieldInfo(argT, string argName) {
>     template FieldInfo(Nullable!argT argDefault = 
> Nullable!argT()) {
>     }
> }
>
> alias processFields(T, string name) =
>     AliasSeq!(FieldInfo!(T, name)());
>

The inner `FieldInfo` template isn't a function, but by adding a 
second set of parentheses after `(T, name)`, you're attempting to 
call it as one.

If you want to instantiate the inner template with its default 
argument, you can use `std.meta.Instantiate`:

alias processFields(T, string name) =
     AliasSeq!(Instantiate!(FieldInfo!(T, name)));