Spurious error with constrained type variables in generic functions

[whoiscc]

Reproduction:

from typing import TypeVar, List


class A:
    pass


class B(A):
    pass


class C:
    pass


LooseType = TypeVar('LooseType', A, B, C)
ListType = List[LooseType]


def foo(xs: ListType[LooseType]) -> None:
    pass


StrictType = TypeVar('StrictType', B, C)


def bar(xs: ListType[StrictType]) -> None:
    foo(xs)

This is denied with error reporting:

test.py:28: error: Argument 1 to "foo" has incompatible type "List[B]"; expected "List[A]"
test.py:28: note: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
test.py:28: note: Consider using "Sequence" instead, which is covariant

Is this really not allowed? If B is not derived from A, then everything is ok.

In my original code, the container of A, B and C is Dict[List[Tuple[LooseType, str]]]. I cannot change List to Sequence because I need append method.

In addition, the notes disappear in complicated case.

[ilevkivskyi]

Thanks for reporting!

Here is a simpler repro (that also is cleaner, since your original repro uses some questionable patterns):

class A:
    pass
class B(A):
    pass
class C:
    pass

LooseType = TypeVar('LooseType', A, B, C)
def foo(xs: List[LooseType]) -> LooseType:
    return xs[0]


StrictType = TypeVar('StrictType', B, C)
def bar(xs: List[StrictType]) -> StrictType:
    foo(xs)
    return xs[0]

It seems to me (but this is a pure guess). This is subtle combination of #4872 and how functions with restricted type variables are checked.

[whoiscc]

Actually I uses StrictType and LooseType in such way:

from typing import TypeVar, List


class A:
    pass


class B(A):
    pass


class C:
    pass


LooseType = TypeVar('LooseType', A, B, C)
ListType = List[LooseType]


def foo(xs: ListType[LooseType]) -> LooseType:
    return xs[0]


StrictType = TypeVar('StrictType', B, C)


def bar(xs: ListType[StrictType]) -> StrictType:
    return foo(xs)

(Only last two lines are changed.)

And this causes one more error with inheritance:

test.py:28: error: Incompatible return value type (got "A", expected "B")

It does not exist neither if B is not derived from A. I thought this is an unrelevant issue and eliminated it from original snippet.