Unions of `TypedDict` turn into intersections in some generic functions

Bug Report
When passing unions of TypedDict to certain generic functions, mypy deduces their type to be the intersection of the TypedDicts, and not the union.

Discovered here.

To Reproduce

from typing import TypedDict, TypeVar

class A(TypedDict):
    shared: str
    different: str

class B(TypedDict):
    shared: str
    different: int
    extra: int

T = TypeVar("T")
def f(a: T, b: T) -> T:
    return b

def g(a: T | None, b: T) -> T:
    return b

c: A | B
d: A | B

r1: A | B = f(c, d)  # passes
r2: A | B = g(c, d)  # expression has type "TypedDict({'shared': str}) [assignment]

Expected Behavior
g(c, d) should have type A | B and not {"shared": str}.

Actual Behavior

mypy_error.py:23: error: Incompatible types in assignment (expression has type "TypedDict({'shared': str})", variable has type "Union[A, B]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.0.0 (compiled: yes)
  • Same error on mypy 0.800, Python 3.10.9
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.10.9

まず issue の最小限の Python 再現コードを実行し、f(c, d) と g(c, d) の推論された型を比較します。TypedDict のユニオンに対する mypy のジェネリック型推論を追跡し、次に例を対象とするリグレッションテストを追加して、g(c, d) が A | B として受け入れられることを確認します。