See python/mypy#606. We allow the following, and they are well-defined:

from typing import TypeVar, Dict
T = TypeVar('T')
U = TypeVar('U')
X = Dict[int, str]
Y = Dict[T, U]

But what does this mean?

Z = Dict[T, int]

In typing.py this currently defines Z as something with two parameters, the second constrained to int or subclasses thereof. But another reasonable interpretation would be that Z has one parameter, corresponding to T only.

The second interpretation is actually more reasonable. (As long as we require that all parameters are either concrete or a simple type variable, not another type expression or alias containing a type variable.)

If we agree, we should define this in the PEP and implement it in typing.py.