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)());

void main(string[] args)
{
    alias Fields = processFields!(int, "x");
}
///

/tmp/temp_7F58BE2F0150.d(12,34): Error: template `runnable.FieldInfo!(int, "x").FieldInfo` cannot deduce function from argument types `!()()`, candidates are:
/tmp/temp_7F58BE2F0150.d(7,5):        `runnable.FieldInfo!(int, "x").FieldInfo(Nullable!int argDefault = Nullable!argT())`
/tmp/temp_7F58BE2F0150.d(16,20): Error: template instance `runnable.processFields!(int, "x")` error instantiating

LDC - the LLVM D compiler (1.11.0):
  based on DMD v2.081.2 and LLVM 6.0.1

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)));