[Issue 22859] Error: forward reference of variable `isAssignableTo` for mutually recursed SumType

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22859

--- Comment #2 from Tomoya Tanjo <ttanjo at gmail.com> ---
Here is the error message caused by the reduced code:

```console
> rdmd playground.d

/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(193):
Error: forward reference of variable `isAssignable`
onlineapp.d(12): Error: template instance
`core.internal.traits.allSatisfy!(isAssignable, None, C1, C2)` error
instantiating
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(193):
Error: template instance `core.internal.traits.F!(C2, C2)` error instantiating
onlineapp.d(7):        instantiated from here: `allSatisfy!(isAssignable, None,
C2)`
```

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