Fixes https://github.com/astral-sh/ty/issues/769.
**Updated:** The preferred approach here is to keep the SemanticIndex
simple (`del` of any name marks that name "bound" in the current scope)
and to move complexity to type inference (free variable resolution stops
when it finds a binding, unless that binding is declared `nonlocal`). As
part of this change, free variable resolution will now union the types
it finds as it walks in enclosing scopes. This approach is still
incomplete, because it doesn't consider inner scopes or sibling scopes,
but it improves the common case.
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Co-authored-by: Carl Meyer <carl@astral.sh>
## Summary
This PR closes https://github.com/astral-sh/ty/issues/238.
Since `DefinitionState::Deleted` was introduced in #18041, support for
the `del` statement (and deletion of except handler names) is
straightforward.
However, it is difficult to determine whether references to attributes
or subscripts are unresolved after they are deleted. This PR only
invalidates narrowing by assignment if the attribute or subscript is
deleted.
## Test Plan
`mdtest/del.md` is added.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Jack O'Connor
Shunsuke Shibayama