Suggestion: Allow free type variables in type variable bounds that permeate beyond

Suppose we have the following code:

T_co = TypeVar("T_co", covariant=True)

class CanProduce(Protocol[T_co]):
    def produce(self) -> T_co:
        ...

T_in = TypeVar("T_in")
U = TypeVar("U")

@dataclass
class Container(Generic[T_in]):
    value: T_in

    def produce_from_value(self: Container[CanProduce[U]]) -> U:
        return self.value.produce()

This code has an undesirable property: Since T_in is invariant, produce_from_value can only be called on instances of Container whose type parameter is exactly CanProduce[U], for some U:

class IntProducer(CanProduce[int]):
    def produce(self) -> int:
        return 42

c: Container[IntProducer] = Container(IntProducer())
c.produce_from_value()  # this produces a type error

It would be ideal if we could communicate that in this method (which need not be an instance method, it could be a discrete function also), the type parameter behaves as though it were covariant, where any subtype of CanProduce[U] is valid.

In fact, there exists a mechanism for doing this in other situations. Consider if we have the following non-generic protocol:

class SupportsIndex(Protocol):
    def __index__(self) -> int:
        ...

We can then make a method index_from_value that works with all subtypes of SupportsIndex:

SI = TypeVar("SI", bound=SupportsIndex)

@dataclass
class Container(Generic[T_in]):
    value: T_in
    
    def index_from_value(self: Container[SI]):
        return self.value.__index__

Now, the following is valid:

c: Container[int] = Container(27)
c.index_from_value()

Unfortunately, this method cannot be used with generic protocols, because the following is invalid:

U = TypeVar("U")
CPU = TypeVar("CPU", bound=CanProduce[U])  # this produces a type error

I suggest altering the restriction to allow this trick to work.

Bắt đầu với các ví dụ generic về protocol, Container và TypeVar trong issue, sau đó so sánh chúng với trường hợp không generic SupportsIndex. Issue chỉ sẵn sàng để thực hiện khi thay đổi restriction được đề xuất và hành vi của nó đối với các subtype được đặc tả đủ rõ ràng để có thể đánh giá.