generics inferred differently when assigning to variable even though the variable's type is the same

[DetachHead]

from typing import Callable, TypeVar

A = TypeVar("A")
B = TypeVar("B")


def func(a: A, b: B, fn: Callable[[A], A]) -> A | B:
 ...


inferred = func(
 1,
 None,
 lambda value: reveal_type(value), # Revealed type is "builtins.int"
)
reveal_type(inferred) # "Union[builtins.int, None]"

explicit: int | None
explicit = func(
 1,
 None,
 lambda value: reveal_type(value), # Revealed type is "Union[builtins.int, None]"
)
reveal_type(explicit) # "Union[builtins.int, None]"

https://mypy-play.net/?mypy=latest&python=3.10&gist=7c2af9e77a6246ee6fb3e5052722eec1

[DetachHead]

a simpler example

from typing import TypeVar

T = TypeVar("T")

def foo(value: list[T]) -> T | int:
 ...


def bar(value: list[T]) -> None:
 # error: Argument 1 to "foo" has incompatible type "List[T]"; expected "List[Union[T, int]]" [arg-type]
 baz: T | int = foo(value)
 
 qux = foo(value)
 quxx: T | int = qux # no error

https://mypy-play.net/?mypy=latest&python=3.10&flags=show-error-context%2Cshow-error-codes%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Clocal-partial-types%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=ea9fa865399112b3d9b5e2c7f2c861e4