Summary

On main we have this behaviour:

from typing import reveal_type
from ty_extensions import is_subtype_of

class Covariant[T]:
    def get(self) -> T:
        raise NotImplementedError

# revealed: `ConstraintSet[never]`
reveal_type(is_subtype_of(Covariant[int | str], Covariant[str] | Covariant[int]))

This is incorrect: due to covariance, the types Covariant[int | str] and Covariant[str] | Covariant[int] are actually equivalent types.

Possibly we could fix this by ensuring that (for covariant and bivariant types) we always eagerly explode unions when creating specialized generics. So, similar to the way the type T & (U | V) can never exist in our type representation (we would always represent the type as (T & U) | (T & V), we would never permit the type Covariant[int | str] to exist in our internal representation (it would always be represented internally as Covariant[str] | Covariant[int]).

The same issue exists for tuples (#493 tracks that)

Version

No response