[Diagnostics] Improve error when type parameters aren't equal

[mdiep]

This improves the error for code like the following:

var a = [1,2,3,4]
var b = a.popFirst()

Before it would give this error:

error: '[Int]' requires the types '[Int]' and 'ArraySlice<Int>' be equivalent to use 'popFirst'
var b = a.popFirst()
          ^

I found that to be very unclear about why that was a requirement.

Now it gives this error, with note about where the method was declared:

error: 'Collection' requires the types '[Int]' and 'Array<Int>.SubSequence' ('ArraySlice<Int>') be equivalent to use 'popFirst'
var b = a.popFirst()
          ^
Swift.Collection:2:37: note: 'popFirst()' declared here
    @inlinable public mutating func popFirst() -> Self.Element?
                                    ^

That seems much clearer to me.

I'm not sure if it's possible to use a locally-built copy of Swift in Xcode, so I'm not sure if that location is enough for tools like Xcode to properly link to the declaration of popFirst.

Resolves SR-7177.

[mdiep]

This ID is not my best naming, but nothing better jumped out at me.

[mdiep]

This is… not as clean as I'd like. I'm very open to suggestions about how it might be improved.

[slavapestov]

@DougGregor might want to take a look.

[xedin]

At least to me new messages don't seem to be much clearer about what is going on especially for.popFirst(), maybe a better solution here would be to print the actual requirement which got broken like we do in other cases e.g. https://github.com/apple/swift/blob/master/test/Constraints/generics.swift#L251, WDYT?

[mdiep]

Hmm… it does seem like it be nice to have parity there 🤔

Unfortunately, I'm not sure it's possible without a somewhat major change to this diagnostic. (I'd love to be wrong on this.)

That information is available on the Requirement. I can get to that through member's GenericContext. It looks like the Constraint is generated from the Requirement, but I don't see a way to connect that Requirement back to the current Constraint to know which of the Requirements failed.

I tried moving the diagnoseArgumentGenericRequirements earlier—to be before this check—but it didn't hit.

It sure seems like there ought to be a way to unify these two things. But there's quite a bit of code between them, and I don't understand enough of the big picture to know how to connect them.

This PR seems like an improvement to me, so I'd probably take this improvement for now. But I'm happy to spend some more time looking—especially if you have any suggestions.

[xedin]

Constraint locator should tell you exactly what requirement is this constraint related to and you can get it by index from generic signature of the context, take a look at how open{Unbound}Generic/openGenericRequirements generates such constraints.

[mdiep]

How does this look?

error: value of type '[Int]' has no member 'popFirst'
var b = a.popFirst()
          ^
Swift.Collection:2:37: note: candidate requires that the types '[Int]' and 'ArraySlice<Int>' be equivalent (requirement specified as 'Self' == 'Self.SubSequence')
    @inlinable public mutating func popFirst() -> Self.Element?
                                    ^

[xedin]

@mdiep I'm not sure what is going on but for some reason I can't see any new changes... But the new node looks great though!

[mdiep]

Okay, just pushed the updated code and tests.

Thanks for the help on this one, @xedin!

[xedin]

LGTM! One last nit request - could you please use clang-format on the commit before we merge this?

[mdiep]

could you please use clang-format on the commit before we merge this?

Done!

[xedin]

@swift-ci please test and merge

[xedin]

@swift-ci please test and merge

[DougGregor]

This looks fantastic, thank you!