Variance inference and subtyping #16

2026-06-20T16:54:56+02:00

HEL commented

2026-06-20 16:54:56 +02:00

This PR implements variance for type variables. To make UX pleasant for Python users who may not be familiar with type theory and variance, this property is inferred from usages of said variables.
It adds a VarianceInferrer class which walks a generic type definition recursively and tracks the polarity of its parameters' use sites, determining the final variance of each type variable.
The subtyping relationship check is also extended to handle comparing two AppliedType instances of the same generic base, taking into account the variance of type variables.

This PR implements variance for type variables. To make UX pleasant for Python users who may not be familiar with type theory and variance, this property is inferred from usages of said variables. It adds a `VarianceInferrer` class which walks a generic type definition recursively and tracks the polarity of its parameters' use sites, determining the final variance of each type variable. The subtyping relationship check is also extended to handle comparing two `AppliedType` instances of the same generic base, taking into account the variance of type variables.

HEL self-assigned this 2026-06-20 16:54:56 +02:00

HEL added 3 commits 2026-06-20 16:54:56 +02:00

feat(checker): infer type variables variance cd8f14153d

feat(checker): use variance in subtype check 9467187313

tests: add tests for variance inference and subtyping 48a20b4aa0

HEL merged commit 12782dda1e into main

2026-06-20 16:55:02 +02:00

HEL deleted branch feat/variance

2026-06-20 16:55:02 +02:00

HEL referenced this issue from a commit

2026-06-20 16:55:03 +02:00

Merge pull request 'Variance inference and subtyping' (#16) from feat/variance into main