Type narrowing does not work when a tuple with union and other type is passed to `isinstance`

[PerchunPak]

Description

isinstance can accept union types as an argument, but when I use a union and another type in the isinstance call, basedpyright fails to narrow types

from typing import reveal_type, TypeAlias

class A: ...
class B: ...
class C: ...

Alias: TypeAlias = A | B

def get_foo() -> Alias | None: ...

foo: Alias | None = get_foo()

if isinstance(foo, Alias):
    # this works, see https://github.com/microsoft/pyright/issues/2565
    reveal_type(foo) # A | B
if isinstance(foo, (Alias, C)):
    # expected A | B | C
    # but got  A | B | None
    reveal_type(foo)

Playground

[DetachHead]

C isn't included in the initial type declaration for foo, so the only way foo can be a C is if it's a subclass of A & C, or B & C. C doesn't appear in the revealed type no matter how you write the isinstance check:

if isinstance(foo, (Alias, C)):
    reveal_type(foo) # revealed: A | B | None
if isinstance(foo, A | B | C):
    reveal_type(foo) # revealed: A | B
if isinstance(foo, (A, B, C)):
    reveal_type(foo)  # revealed: A | B

which makes sense to me, because isinstance would narrow foo to (A | B) | ((A | B) & C) which simplifies to just A | B.

but i agree the None should definitely get narrowed out here

[PerchunPak]

Oh, you are right. In my real use case, the Alias type looks like this:

from typing import reveal_type, TypeAlias

class Red: ...
class Green: ...
class Bold: ...
class Italic: ...

Color: TypeAlias = Red | Green
Style: TypeAlias = Bold | Italic
Formatting: TypeAlias = Color | Style

def get_foo() -> Formatting: ...
foo: Formatting = get_foo()

if isinstance(foo, (Color, Bold)):
    # expected Red | Green | Bold
    # but got  Red | Green | Bold | Italic
    reveal_type(foo)