issue with intersection compatibility with nested union

```py
from __future__ import annotations

class A: ...
class B: ...
class C: ...

a: A & (B | C)
b: A & (B | C) = a # Incompatible types in assignment (expression has type "A & B | C", variable has type "A & B | C")
```

Start with the Python reproducer in the issue and run it through basedmypy to confirm the incompatible-assignment diagnostic. Trace the type-checking path for intersections nested inside unions, then add a regression test using this example; done means the compatible assignment is accepted without the diagnostic.

Written by the indexing model from the issue text.