List and other invariant types should behave covariantly on runtime typechecking

[Stewori]

For runtime typechecking deep_type is used to determine the PEP 484 type of a given object. Then is_subtype is used to detect if the type is acceptable. E.g. [2] is considered List[int] by deep_type although it could also be List[float] or List[Union[int, whatever]].

Solution idea:
Widen the use of Empty which is currently used to handle [] and {}, etc.
It would be renamed to Sampled or so and serve as a marker type for is_subtype to check its contained type, e.g. Sampled[List[int]] in covariant manner even if it is actually invariant.

[jolaf]

Sounds cool, thank you!

[jolaf]

I wonder if this Sampled[] and Empty[] things should be visible in TypeWarning printout when actual type mismatch occurs.

Probably they are better to be removed at printing to not confuse the user.

[Stewori]

Yes that's right. Or at least invisible unless some debug flag is set. I would cure this in type_str. However this won't fix before Python 3.7 compatibility is achieved because it requires some broader restructuring. I have rather limited capacity to tackle these bigger changes.