Mypy doesn't understand structural subtyping for nested TypedDicts

Bug Report

When a structural subtyping relation between two "inner" TypedDicts should cause "outer" TypedDicts that contain them (and only them) to be subtypes of each other as well, Mypy fails to understand this and reports that the outer subtype is incompatible with the outer supertype.

To Reproduce

from typing import TypedDict

class InnerSuper(TypedDict):
  x: str

class InnerSub(TypedDict):
  x: str
  y: int

class OuterSuper(TypedDict):
  inner: InnerSuper

class OuterSub(TypedDict):
  inner: InnerSub

# Mypy understands that InnerSub is a structural subtype of InnerSuper...
inner_sub: InnerSub
inner_super: InnerSuper = inner_sub  # no type error

# ... but not that OuterSub is a structural subtype of OuterSuper:
outer_sub: OuterSub
outer_super: OuterSuper = outer_sub  # type error

Expected Behavior

No type errors are reported by Mypy because OuterSub is a structural subtype of OuterSuper.

Actual Behavior

Mypy reports an error:

example.py:22: error: Incompatible types in assignment (expression has type "OuterSub", variable has type "OuterSuper")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Mypy version used: 1.11.2
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: 3.10.12

Now that it has turned out this is expected behavior (see comment below), a few keywords to let other people find this issue more easily: covariance, variance, invariance, covariant

python / mypy

Mypy doesn't understand structural subtyping for nested TypedDicts #17754