[Issue 21891] Cannot alias every kind that can be passed via variadic templates

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21891

Paul Backus <snarwin+bugzilla at gmail.com> changed:

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                 CC|                            |snarwin+bugzilla at gmail.com

--- Comment #2 from Paul Backus <snarwin+bugzilla at gmail.com> ---
(In reply to Andrei Alexandrescu from comment #0)
> So in fact 1 is bindable to an alias but somehow not in all contexts!

Specifically: an expression can bind to a template alias parameter, but is not
allowed on the right-hand side of an alias declaration.

The reason `alias Y = X;` works but `alias Y = X[0];` doesn't is that `X` by
itself on the right-hand side is parsed as a symbol, not an expression. The
variadic parameter is a distraction.

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