Broken reachability checks in match-case

[denballakh]

Bug Report
I have two small similar examples for you:
1:

def f(x: object, y: object) -> int | None: # Missing return statement
    match x, y:
        case int(), int() if True:
            return 42
        case _:
            return None

2:

def f(x: object, y: object) -> int | None: # Missing return statement
    match x, y:
        case _, _ if True:
            return 42
        case _:
            return None

Expected Behavior

1. No Missing return statement error, because code after match statement is unreachable.
2. No Missing return statement error and error statement is unreachable on line case _:

Actual Behavior

1. Error Missing return statement.
2. Error Missing return statement. No statement is unreachable error on line case _:

Error Missing return statement will disappear if i remove if True: condition (in both cases).
Error statement is unreachable will appear if i replace case _, _: with case _:. Removing condition doesnt help.

Your Environment

• Mypy version used: mypy 0.982 (compiled: no)
• Mypy command-line flags: no
• Mypy configuration options from mypy.ini: no
• Python version used: 3.11
• Windows 10

And now for something a bit different

def f(x: object, y: object) -> int | None:
    match x, y:
        case int(), int():
            return x + y # Unsupported left operand type for + ("object")
        case _:
            return None

Mypy cannot narrow types of tuple elements.

[tmke8]

Regarding your "And now for something a bit different", there is an issue for that: #12364.

[erictraut]

This is a duplicate of #12534.