Re: Why this template code does not compile?

[Paul Backus via Digitalmars-d-learn]

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)));