Need a convenient syntax for invoking nested eponymous templates

[dlangBugzillaToGithub]

Yuxuan Shui (@yshui) reported this on 2018-08-05T00:13:40Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19139

CC List

• Simen Kjaeraas

Description

Currently, given:

template A() {
    template A() {
        enum A = 1;
    }
}

This:

A!()!()

does not compile (multiple ! arguments are not allowed).

Neither does this (C style cast illegal):

(A!())!()

The only way to invoke the nested template seems to be:

alias B = A!();
B!()

This is less than ideal.

[dlangBugzillaToGithub]

simen.kjaras commented on 2018-08-05T09:26:22Z

import std.meta : Instantiate;

template A() {
    template A() {
        enum A = 1;
    }
}

unittest {
    enum i = Instantiate!(A!());
}

Now, that shows it's possible in the language to work around this issue. The real issue however, is that multiple ! arguments are not allowed.

From what I can gather on the forum, the reason is a perceived ambiguity, in that F!T!int could mean F!(T!int) or (F!T)!int. The language already provides tools to disambiguate between them - the first example is how you'd invoke it that way, and the other example is simply impossible in the language right now.