[ty] Fix bidirectional inference with PEP 695 union type aliases#22988
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[ty] Fix bidirectional inference with PEP 695 union type aliases#22988
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When a PEP 695 type alias that expands to a union (like `type MaybeList[T] = list[T] | T`) is used as a type annotation, bidirectional type inference was failing to infer the correct specialization for generic calls. The issue was in `ArgumentTypeChecker::infer_specialization` where the type context was used directly without expanding type aliases. When the type context is a `Type::TypeAlias`, the `filter_union` and `class_specialization` calls would fail because the type alias wasn't being expanded to its value type first. The fix expands type aliases to their value types before using them for specialization inference, allowing union members to be correctly filtered and matched. Fixes astral-sh/ty#2682 https://claude.ai/code/session_01XvWDzFJcmPfrXQRJdmpMBb
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It might be worth checking other uses of filter_union to see if we rely on type aliases being expanded. Ideally filter_union would expand type aliases internally if needed, but I think that would break the current generics solver.
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This change modifies `filter_union` to resolve type aliases before checking if the type is a union. This allows `filter_union` to properly handle cases where a type alias resolves to a union type. With this change, the explicit `resolve_type_alias` call in `bind.rs` (added in #22988) is now redundant and has been removed. Added additional test cases for union type aliases with: - TypedDict dict literal inference - TypedDict dict() call inference - Set literal inference https://claude.ai/code/session_01Fsp2dSw6mnTBV1M2Fnf6gt
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Summary
Fixes astral-sh/ty#2682
Fix bidirectional type inference when PEP 695 type aliases expand to union types. Previously, when a type alias like
type MaybeList[T] = list[T] | Twas used as a type annotation, the specialization inference would fail to correctly infer type parameters from the assigned value.The issue was that type aliases were not being expanded to their value types before filtering union members for specialization inference. This prevented the type checker from identifying class instances within the expanded union that could be used to infer generic type parameters.
The fix expands type aliases to their underlying value types before performing the union filtering and reverse type inference. This allows bidirectional inference to work correctly with union type aliases while maintaining proper variance handling.
Test Plan
Added test cases in
crates/ty_python_semantic/resources/mdtest/generics/pep695/aliases.md.