Pyre cannot narrow literal types

[vthemelis]

Pyre Bug

Bug description

Pyre cannot reason about narrowing of literal types:

from typing import Literal, assert_never

def foo(x: Literal["a", "b"]) -> str:
    if x == "a":
        return "a"
    if x == "b":
        return "b"
    else:
        assert_never(x)

This should obviously type check (and it does for pyright)

See Pyre playground