[Issue 18557] Types with 0 size should not be usable as aa key types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18557

Ketmar Dark <ketmar at ketmar.no-ip.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ketmar at ketmar.no-ip.org

--- Comment #3 from Ketmar Dark <ketmar at ketmar.no-ip.org> ---
this patch breaks Variant: it is legal to use `This[This]` as a placeholder
type in Variant, and with the patch applied that code doesn't compiles anymore
('cause `This` is defined as `struct This;`).

adding real definition to `This` doesn't help too, 'cause then dmd errored with
"recursive template expansion".

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