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

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.

Start by reproducing the issue's Python example and compare the two reveal_type results with the expected narrowing. Read the linked comment on PR #18180 for background, then trace the type-checking path responsible for assignment-based narrowing. Done means the final revealed type is int | None without regressing the demonstrated behavior.