[red-knot] Acknowledge that T & anything is assignable to T
#17413
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sharkdp
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This reworks the assignability/subtyping relations a bit to handle typevars better: 1. For the most part, types are not assignable to typevars, since there's no guarantee what type the typevar will be specialized to. 2. An intersection is an exception, if it contains the typevar itself as one of the positive elements. This should fall out from the other clauses automatically, since a typevar is assignable to itself, and an intersection is assignable to something if any positive element is assignable to that something. 3. Constrained typevars are an exception, since they must be specialized to _exactly_ one of the constraints, not to a _subtype_ of a constraint. If a type is assignable to every constraint, then the type is also assignable to the constrained typevar. We already had a special case for (3), but the ordering of it relative to the intersection clauses meant we weren't catching (2) correctly. To fix this, we keep the special case for (3), but fall through to the other match arms for non-constrained typevars and if the special case isn't true for a constrained typevar. Closes #17364
dcreager
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* main: (44 commits) [`airflow`] Extend `AIR311` rules (#17422) [red-knot] simplify union size limit handling (#17429) [`airflow`] Extract `AIR311` from `AIR301` rules (`AIR301`, `AIR311`) (#17310) [red-knot] set a size limit on unions of literals (#17419) [red-knot] make large-union benchmark slow again (#17418) [red-knot] optimize building large unions of literals (#17403) [red-knot] Fix comments in type_api.md (#17425) [red-knot] Do not assume that `x != 0` if `x` inhabits `~Literal[0]` (#17370) [red-knot] make large-union benchmark more challenging (#17416) [red-knot] Acknowledge that `T & anything` is assignable to `T` (#17413) Update Rust crate clap to v4.5.36 (#17381) Raise syntax error when `\` is at end of file (#17409) [red-knot] Add regression tests for narrowing constraints cycles (#17408) [red-knot] Add some knowledge of `__all__` to `*`-import machinery (#17373) Update taiki-e/install-action digest to be7c31b (#17379) Update Rust crate mimalloc to v0.1.46 (#17382) Update PyO3/maturin-action action to v1.49.1 (#17384) Update Rust crate anyhow to v1.0.98 (#17380) dependencies: switch from `chrono` to `jiff` Update Rust crate bstr to v1.12.0 (#17385) ...
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This reworks the assignability/subtyping relations a bit to handle typevars better:
For the most part, types are not assignable to typevars, since there's no guarantee what type the typevar will be specialized to.
An intersection is an exception, if it contains the typevar itself as one of the positive elements. This should fall out from the other clauses automatically, since a typevar is assignable to itself, and an intersection is assignable to something if any positive element is assignable to that something.
Constrained typevars are an exception, since they must be specialized to exactly one of the constraints, not to a subtype of a constraint. If a type is assignable to every constraint, then the type is also assignable to the constrained typevar.
We already had a special case for (3), but the ordering of it relative to the intersection clauses meant we weren't catching (2) correctly. To fix this, we keep the special case for (3), but fall through to the other match arms for non-constrained typevars and if the special case isn't true for a constrained typevar.
Closes #17364