[Issue 19605] invalid forward reference error when getting the `.sizeof` of an opaque enum

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19605

--- Comment #3 from Basile-z <b2.temp at gmx.com> ---
This one might be invalid actually. The implementation of the fix is
problematic at least.

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