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

[schuelermine]

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.

[smheidrich]

Related: #1226