Unpacking doesn't do proper type narrowing

[RbCaVi]

Bug Report

When you unpack a tuple with a union type after an if statement that causes narrowing, the unpacked elements have a different type than the elements of the tuple.

To Reproduce

Playground

Expected Behavior

I expected the revealed types of x[0] and y to be the same as each other in all three cases.

Actual Behavior

../test.py:9: note: Revealed type is "Union[tuple[builtins.int], tuple[builtins.float]]"
../test.py:10: note: Revealed type is "builtins.float"
../test.py:11: note: Revealed type is "Union[builtins.int, builtins.float]"

../test.py:20: note: Revealed type is "Union[tuple[Literal['X']], tuple[test.S]]"
../test.py:21: note: Revealed type is "test.S"
../test.py:22: note: Revealed type is "Union[Literal['X'], test.S]"

../test.py:30: note: Revealed type is "tuple[Union[Literal['X'], test.S]]"
../test.py:31: note: Revealed type is "test.S"
../test.py:32: note: Revealed type is "Union[Literal['X'], test.S]"

Your Environment

• Mypy version used: Github (mypy 1.11.0+dev.35fbd2a852be2c47e8006429afe9b3ad4b1bfac2 (compiled: no))
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12

[rtumin2]

I believe this is another example

from typing import Union

my_type = Union[
    tuple[float, None],
    tuple[None, float],
]


def f(x: my_type):
    a, b = x
    if a is not None:
        return a*2
    else:
        return b*2

This gives

test.py:14: error: Unsupported operand types for * ("None" and "int")  [operator]
test.py:14: note: Left operand is of type "float | None"

Compare to the following, in which the false positive does not occur:

from typing import Union

my_type = Union[
    tuple[float, None],
    tuple[None, float],
]


def f(x: my_type):
    if x[0] is not None:
        return x[0]*2
    else:
        return x[1]*2