https://d.puremagic.com/issues/show_bug.cgi?id=12411

           Summary: New eponymous template syntax could support nested
                    eponymous templates
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-03-19 01:42:34 PDT ---
Example code:

-----
import std.typetuple;

enum canAppend(R) = anySatisfy!(canAppendRange!R, Types);

private alias Types = TypeTuple!(int, float);

// problem: can't use new-style eponymous syntax for nexted templates
private template canAppendRange(R)
{
    enum canAppendRange(T) = is(typeof({ R r = void; T t = void; r ~= t; }));
}

void main()
{
    static assert(canAppend!(int[]));
    static assert(canAppend!(float[]));
    static assert(!canAppend!(byte[]));
}
-----

Notice how 'canAppendRange' has to use the old-style eponymous syntax in order for it to be usable with partial instantiation in the call to anySatisfy.

Perhaps a somewhat reasonable enhancement would be to allow nested eponymous syntax:

-----
enum canAppendRange(R)(T) = is(typeof({ R r = void; T t = void; r ~= t; }));
-----

A simplified example test-case:

-----
// old-style
template isEqual(T1)
{
    template isEqual(T2)
    {
        enum bool isEqual = is(T1 == T2);
    }
}

// new-style
enum isEqual(T1)(T2) = is(T1 == T2);

void main()
{
    alias isInt = isEqual!int;
    static assert(isInt!int);
}
-----

The whole idea is to allow new syntax to allow easier writing of templates which can be partially instantiated.

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