Assign it to a variable and return it once.

from typing import Literal, assert_never, reveal_type

def foo(x: Literal["a", "b"]):
    match x:
        case "a":
            result = 42
        case "b":
            result = "string"
        case _:
            assert_never(x)
    return result

reveal_type(foo)  # Pyright infers: (x: Literal['a', 'b']) -> Literal[42, 'string']