[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

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Spec: https://github.com/dlang/dlang.org/pull/2265

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