Why a template with Nullable does not compile?

[H. S. Teoh]

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);


T

-- 
It won't be covered in the book. The source code has to be useful for something, after all. -- Larry Wall