0.58.0 regression: false positive `bad-return`

[jorenham]

The following typestats code started reported an error after upgrading pyrefly from 0.57.1 to 0.58.0 (jorenham/typestats#335):

def key(self, index: int, /) -> int | str:
    match self.kind:
        case ParamKind.POSITIONAL_ONLY:
            return index
        case ParamKind.KEYWORD_ONLY | ParamKind.POSITIONAL_OR_KEYWORD:
            return self.name
        case ParamKind.VAR_POSITIONAL:
            return "*"
        case ParamKind.VAR_KEYWORD:
            return "**"

where self.kind: ParamKind is an enum defined as follows:

class ParamKind(StrEnum):
    # matches inspect.Parameter.kind
    POSITIONAL_ONLY = "positional-only"
    POSITIONAL_OR_KEYWORD = "positional or keyword"
    VAR_POSITIONAL = "variadic positional"
    KEYWORD_ONLY = "keyword-only"
    VAR_KEYWORD = "variadic keyword"

ERROR Function declared to return `int | str`, but one or more paths are missing an explicit `return` [bad-return]
   --> src/typestats/analyze.py:222:37
    |
222 |     def key(self, index: int, /) -> int | str:
    |                                     ^^^^^^^^^
    |
 INFO 1 error (2 suppressed)
error: Recipe `typecheck` failed on line 13 with exit code 1

See https://github.com/jorenham/typestats/actions/runs/23535046261/job/68508003477?pr=335 for details.

AS you cans see, this method only returns int or str, because all enum variants are handled by the match statement, so it will never return None.

If I add an additional case _: assert_never(self.kind), the error disappears (and no new ones appear either).

[jorenham]

Oh sorry, this might actually be a dupe of #2894
Feel free to close this

[superbobry]

I should've mentioned that #2894 is for 0.57.1.