[ty] Avoid cross-TypeVar leakage in generic inference#26099
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 94.37%. The percentage of expected errors that received a diagnostic held steady at 89.00%. The number of fully passing files held steady at 94/134. |
Memory usage reportMemory usage unchanged ✅ |
|
| Lint rule | Added | Removed | Changed |
|---|---|---|---|
invalid-assignment |
1 | 5 | 0 |
| Total | 1 | 5 | 0 |
Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.
Raw diff:
Expression (https://github.com/cognitedata/Expression)
+ tests/test_compose.py:21:16 error[invalid-assignment] Object of type `(_A@compose, /) -> _A@compose` is not assignable to `(int, /) -> int`
django-modern-rest (https://github.com/wemake-services/django-modern-rest)
- dmr/security/jwt/blocklist/models.py:30:23 error[invalid-assignment] Object of type `CharField[int | str | Combinable, int | str | Combinable]` is not assignable to `CharField[int | str | Combinable, str]`
- dmr/security/jwt/blocklist/models.py:34:34 error[invalid-assignment] Object of type `DateTimeField[str | datetime | Combinable, str | datetime | Combinable]` is not assignable to `DateTimeField[str | datetime | Combinable, datetime]`
- dmr/security/jwt/blocklist/models.py:35:34 error[invalid-assignment] Object of type `DateTimeField[str | datetime | Combinable, str | datetime | Combinable]` is not assignable to `DateTimeField[str | datetime | Combinable, datetime]`
- dmr/security/jwt/blocklist/models.py:38:34 error[invalid-assignment] Object of type `DateTimeField[str | datetime | Combinable, str | datetime | Combinable]` is not assignable to `DateTimeField[str | datetime | Combinable, datetime]`
meson (https://github.com/mesonbuild/meson)
- mesonbuild/compilers/compilers.py:104:44 error[invalid-assignment] Object of type `dict[str, str]` is not assignable to `Mapping[str, Literal["c", "cpp", "cuda", "fortran", "d", ... omitted 11 literals]]`
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Summary
Generic return-type context can introduce relationships between type variables while solving a call. We need those relationships during constraint solving, but we previously reused them as concrete preferred types during argument inference.
For example, inferring
dict(zip(keys, values))againstMapping[float, int]can derive a key solution containing the value TypeVar and a value solution containing the key TypeVar. Treating those intermediate relationships as final solutions causes bothdictparameters to widen toint | float. This fails onmain:This projects inferable TypeVar artifacts within each source binding context before using solutions as preferred types. Relationships across separate generic contexts remain intact, preserving constructor
selfremapping and similar inference.