Partial type that is "initialised" via assignment is only temporarily narrowed by `isinstance`

[tyralla]

In the following example, the last revealed type could/should be Union[builtins.int, None]:

def f() -> bool: 
 return True

u: int | tuple[int]
x = None
if f():
 x = u
 if isinstance(x, tuple):
 x = x[0]
 reveal_type(x) # note: "builtins.int"
reveal_type(x) # note: Revealed type is "Union[builtins.int, Tuple[builtins.int], None]"

See this comment for background information.