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

Start by running the minimal Python reproducer from the issue and compare the inferred types of f(c, d) and g(c, d). Trace mypy's generic type inference for TypedDict unions, then add a regression test covering the example and confirm that g(c, d) is accepted as A | B.