Incorrect constraint inference for unions with nested generics

🐛 Bug Report

From my search, it looks like an issue similar to #6417, but happening to unions of generic and nested generic.
mypy has troubles with inferring a proper constraint and passing a nested generic to function that should accept such nested generic fails.
The issue can be worked around with @overloads.

To Reproduce

Minimum reproduction:

from typing import List, TypeVar, Union

_T = TypeVar("_T")


def func(param: Union[List[_T], List[List[_T]]]) -> _T:
    ...

func([1])
func([[1]])

A more real-live example of where this is an issue is tornado's gen function (overloads there are the part of workaround for this issue):
https://github.com/tornadoweb/tornado/pull/2909/files

Expected Behavior

No errors.

Actual Behavior

main.py:10: error: Argument 1 to "func" has incompatible type "List[List[int]]"; expected "Union[List[<nothing>], List[List[<nothing>]]]"

Your Environment

• Mypy version used: 0.781
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8.5
• Operating system and version: Windows 10

Commencez par la reproduction minimale dans main.py et vérifiez le comportement différent de func([1]) et func([[1]]) avec mypy. Suivez le chemin d’inférence des contraintes pour Union[List[_T], List[List[_T]]] et comparez-le à l’issue connexe #6417 ainsi qu’au contournement avec overload dans la pull request Tornado liée. C’est terminé lorsque les deux appels passent la vérification de types sans erreur.