Why a template with Nullable does not compile?

[Victor Porton]

On Tuesday, 12 March 2019 at 16:20:11 UTC, H. S. Teoh wrote:
> On Tue, Mar 12, 2019 at 03:26:05PM +0000, Victor Porton via 
> Digitalmars-d-learn wrote: [...]
>> On Tuesday, 12 March 2019 at 09:05:36 UTC, Nicholas Wilson 
>> wrote:
> [...]
>> > template FieldInfo(T) {
>> >     template FieldInfo(Nullable!(T) default_)
>> >     {
>> >         enum FieldInfo = 0;
>> >     }
>> > }
>> > 
>> > seems to work, but I can't seem to instantiate one of it.
>> 
>> Why you use the same name "FieldInfo" for both the template 
>> and its subtemplate? Does it make some sense?
>
> This is a D idiom called the "eponymous template".  Whenever 
> the template contains a member of the same name as the 
> template, it's an eponymous template, and you can refer 
> directly to the member by the template name, rather than using 
> templateName.memberName.
>
> For example, a template function is usually written like this:
>
> 	ReturnType myFunc(TemplateArgs...)(RuntimeArgs args...)
> 	{
> 		... // implementation here
> 	}
>
> This is actually shorthand for the eponymous template:
>
> 	template myFunc(TemplateArgs...)
> 	{
> 		ReturnType myFunc(RuntimeArgs args...)
> 		{
> 			... // implementation here
> 		}
> 	}
>
> Similarly, when you write:
>
> 	enum isInputRange(T) = hasMember!(T, empty) && ...
>
> that's actually shorthand for:
>
> 	template isInputRange(T)
> 	{
> 		enum isInputRange = hasMember!(T, empty) && ...
> 	}
>
> The eponymonus template idiom allows you to use a single name 
> to refer to both the template and the member. Without this 
> idiom, you'd have to use the very verbose notation:
>
> 	static if (isInputRange!T.isInputRange) ...
>
> or
>
> 	auto retval = myFunc!(A, B, C).myFunc(1, 2, 3);

I know what is eponymous template. But how it behaves when the 
eponymous member inside itself is also a template? How to 
instantiate it? (provide please an example how to instantiate)