Fix list concatenation under contextual list hint #2142
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Fixes facebook#2141. When a list[T] contextual hint is available (e.g. assignment/return annotation) and both operands of `+` are fresh list-producing expressions, pass the hint into both operands so `[A()] + [B()]` can be inferred as `list[Base]` in a `list[Base]` context. Keeps `list` invariant and preserves `reveal_type([A()] + [B()]) == list[A | B]` without context.
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Fixes #2141.
What
When a
list[T]contextual hint is available (assignment/return annotation) and both operands of+are fresh list-producing expressions (list literal or list comprehension), propagate the hint into both operands.This makes cases like:
l2: list[Base] = [A()] + [B()]def f() -> list[Base]: return [A()] + [B()]work without making
listcovariant globally.Why
List literals already use contextual typing via
decompose_list, but+previously inferred operands without passing the hint (except[X] * int). That caused[A()] + [B()]to be inferred aslist[A | B]and fail againstlist[Base]due to invariance.Tests
l2and ensuring no-contextreveal_type([A()] + [B()])remainslist[A | B].Notes
The hint is only propagated when both operands are fresh to avoid masking invariance errors for expressions involving non-fresh operands.