Definition of `str.split`, `str.rsplit` and `str.splitlines` results in false positive errors

I recently fixed a bug in pyright that caused a false negative when attempting to assign a list[LiteralString] to a list[str]. As we know, the type parameter for list is invariant, so these types are not consistent.

from typing import LiteralString
v1: list[LiteralString] = ["a", "b"]
v2: list[str] = v1  # Should be a type violation

With this bug fix now in place, the definitions for the methods str.split, str.rsplit and str.splitlines are problematic. These three methods are defined using overloaded signatures, and the first overload returns list[LiteralString]. This means the following code now results in a type violation.

def process(x: list[str]) -> str:
    return " ".join(x)

process("1,2,3".split(","))  # Type violation

This is going to generate a lot of new type errors in existing code bases.

I initially attempted to work around this by adding a heuristic in pyright's overload matching logic so it deprioritized the first overload in this case, but this heuristic broke other code and really isn't sound. I've therefore decided to remove the heuristic.

I don't have any good ideas about how to fix this, but I wanted to open the issue to start the discussion to see if anyone had a suggestion. If we're going to fix it, I think the fix needs to be in the typeshed stubs. I don't see a way to fix this in a type checker — at least while remaining true to the type system.

One possible solution is to omit the LiteralString overloads for these three methods. This would eliminate the type violation error in the common case, but it would introduce new errors in cases where callers expect to see a return result of list[LiteralString].

Note that this doesn't currently affect mypy users because mypy has not implemented support for LiteralString.

まず、str.split、str.rsplit、str.splitlines に対する typeshed のオーバーロードされた定義を確認し、それらの LiteralString の戻り値型に注目します。issue の例を使ってリストの不変性エラーを再現し、議論されている代替案を評価します。妥当な LiteralString の結果を損なうことなく誤検出を解消できれば完了です。