Can't resolve generic type from return type of callable passed as (default) argument?

UPDATE: Probably a dup of #3737 and #10504.

What I want to do is define a function that takes a callable that returns a generic as an argument where that generic influences the return type of the function. Something like this:

def doit(constructor: Callable[..., T]) -> List[T]:
    ...

I can't seem to figure out a way to get that done, though, especially when I am supplying a default argument for the callable. Here are my test cases:

from typing import Callable, List, Protocol, TypeVar

_T = TypeVar("_T")

# error: Incompatible default for argument "constructor" (default has type "Type[str]", argument has type "Callable[[int], _T]")
def generate(n: int, constructor: Callable[[int], _T] = str) -> List[_T]:
    return [constructor(i) for i in range(n)]

# error: Need type annotation for "res" (hint: "res: List[<type>] = ...")
res = generate(5)  # res should be of type List[str]
assert res == ["0", "1", "2", "3", "4"]
assert generate(5, float) == [0.0, 1.0, 2.0, 3.0, 4.0]

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

class Constructor(Protocol[_T_co]):
    def __call__(self, n: int) -> _T_co:
        ...

def generate2(n: int, constructor: Constructor[_T] = str) -> List[_T]:
    return [constructor(i) for i in range(n)]

# error: Need type annotation for "res2" (hint: "res2: List[<type>] = ...")
res2 = generate2(5, float)  # res2 should be of type List[float]
assert res2 == [0.0, 1.0, 2.0, 3.0, 4.0]

Runtime:

% python --version
Python 3.9.6
% python test.py
% mypy --version
mypy 0.910
% mypy --config-file=/dev/null test.py
/dev/null: No [mypy] section in config file
test.py:6: error: Incompatible default for argument "constructor" (default has type "Type[str]", argument has type "Callable[[int], _T]")
test.py:10: error: Need type annotation for "res" (hint: "res: List[<type>] = ...")
test.py:24: error: Need type annotation for "res2" (hint: "res2: List[<type>] = ...")
Found 3 errors in 1 file (checked 1 source file)

I tried searching for a similar issue, but was unable to find what I was looking for.

The reproduction is in test.py; first review the related issues #3737 and #10504, then run mypy --config-file=/dev/null test.py. Done means the callable default is accepted and the generate calls infer List[str] and List[float] without the reported errors.