TypedDict unpacking should be checked more strictly

[Michael0x2a]

I'm not sure if this should be classified as a missing feature or a bug, but the behavior of the following program is surprising to me:

from mypy_extensions import TypedDict

def foo(x: int, y: int) -> None: pass

OkDict = TypedDict('OkDict', {'x': int, 'y': int})
BadDict1 = TypedDict('BadDict1', {'x': int})
BadDict2 = TypedDict('BadDict2', {'x': int, 'y': str})

a: OkDict
b: BadDict1
c: BadDict2

foo(**a)  # Type checks, as expected
foo(**b)  # Type checks; should be an error
foo(**c)  # Type checks; should be an error

Mypy currently reports no errors with this program; I think we should report an error with the last two calls.

(I'm assuming we're currently ignoring the 'shape' of the TypedDicts and are handling them in the same way we handle Dict?)

[ilevkivskyi]

I would rather classify this as a feature (especially taking into account that TypedDict still doesn't have its PEP). Anyway, I believe this is something important to have.