Improve Step 5 of overload call evaluation.

[rchen152]

Makes two modifications to step 5 of overload call evaluation:

1. When materializing arguments to check whether we can eliminate overloads, skip arguments that have the same parameter type in all overloads.
2. When we still have multiple overloads after the materialization filter, try to find a return type that all materializations of all other return types are assignable to.

Updates the conformance tests as well. All type checkers already do (1). Mypy and pyrefly pass the conformance test for (2). I believe mypy has a heuristic that approximates the new rule; pyrefly implements it exactly.

[carljm]

I think it's preferable to keep PRs updating type-checker versions separate from PRs implementing conformance suite and spec changes.

[JelleZijlstra]

I think this is correct but there's some subtlety in the wording that we should cover in the test cases:

• "The parameter types corresponding to this argument". That may be a very different parameter in each of the overloads; e.g. maybe one takes *args and the other has explicit arguments. So my reading is that to check this, you have to create for each overload a mapping between argument and parameter type, and consult that. What is the parameter type corresponding to an unpacked argument, though?
• "the remaining overloads". So we only check this overload and all following overloads, not ones above it?

[rchen152]

Great points, thanks!

• I added a clarification about unpacked arguments and some more tests.
• Ah, I meant all of the candidate overloads that are remaining at the beginning of Step 5, not just the overloads following the one that we're checking. I replaced "remaining overloads" with "candidate overloads" - hopefully that's clearer.

[rchen152]

I think it's preferable to keep PRs updating type-checker versions separate from PRs implementing conformance suite and spec changes.

Point taken, sorry! I was being a bit sloppy because I didn't want to open two PRs XD

Version update here: #2254

[rchen152]

Thanks for the feedback! I believe I've addressed all the comments.

[davidhalter]

Thanks Rebecca for looking into this. I think we really need changes here. Please give me a few days to respond before you call for a vote. I have a few more general thoughts.