[Issue 22184] std.sumtype.SumType with mutual recursion causes a forward reference of variable `isHashable`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22184

--- Comment #1 from Tomoya Tanjo <ttanjo at gmail.com> ---
It works if we declare the classes in the following order: C, A and B.

run.dlang.io: https://run.dlang.io/is/qRsc7e

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