[Issue 12615] Warn against, and then deprecate old alias syntax

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12615

--- Comment #21 from Mike Franklin <slavo5150 at yahoo.com> ---
(In reply to Mike Franklin from comment #20)

> Find a solution to that and we can finally use the assignment syntax for alias 
> this.

A proposal to resolve the issue was presented at
https://github.com/dlang/dmd/pull/1341#issuecomment-34845297

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