[SR-10089] Combining CaseIterable and Strideable seems impossible.

[swift-ci]

Previous ID	SR-10089
Radar	None
Original Reporter	Jessy (JIRA User)
Type	Bug

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, RunTimeCrash
Assignee	None
Priority	Medium

md5: 4e2939aa390658c94ca485ef4e84a78e

Issue Description:

I think CaseIterable is calling distance(to:) behind the scenes?

[belkadan]

Reproduced, though I'm confused about this. cc brentdax (JIRA User), @jtbandes, CodaFi (JIRA User)

[beccadax]

The symptom is infinite recursion that looks like this:

    frame #​52829: 0x0000000100001603 Contents`CaseStrideable.distance(other=a, self=a) at Contents.swift:7:33
    frame #&#8203;52830: 0x0000000100002202 Contents`protocol witness for Strideable.distance(to:) in conformance Alphabet at <compiler-generated>:0
    frame #&#8203;52831: 0x00007fff6ec5126b libswiftCore.dylib`static (extension in Swift):Swift.Strideable.== infix(A, A) -> Swift.Bool + 219
    frame #&#8203;52832: 0x0000000100001ef6 Contents`protocol witness for static Equatable.== infix(_:_:) in conformance Alphabet at <compiler-generated>:0
    frame #&#8203;52833: 0x00007fff6eb2de99 libswiftCore.dylib`(extension in Swift):Swift.Collection< where A.Element: Swift.Equatable>.firstIndex(of: A.Element) -> Swift.Optional<A.Index> + 905
    frame #&#8203;52834: 0x0000000100002776 Contents`partial apply for Collection<>.firstIndex(of:) at <compiler-generated>:0
    frame #&#8203;52835: 0x0000000100001813 Contents`thunk for @escaping @callee_guaranteed (@in_guaranteed A) -> (@out Int?) at <compiler-generated>:0
    frame #&#8203;52836: 0x0000000100001894 Contents`partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed A) -> (@out Int?) at <compiler-generated>:0
    frame #&#8203;52837: 0x00000001000018ba Contents`thunk for @callee_guaranteed (@in_guaranteed A) -> (@unowned Int?, @error @owned Error) at <compiler-generated>:0
    frame #&#8203;52838: 0x0000000100001998 Contents`partial apply for thunk for @callee_guaranteed (@in_guaranteed A) -> (@unowned Int?, @error @owned Error) at <compiler-generated>:0
    frame #&#8203;52839: 0x00007fff6ec34299 libswiftCore.dylib`(extension in Swift):Swift.Sequence.compactMap<A>((A.Element) throws -> Swift.Optional<A1>) throws -> Swift.Array<A1> + 761
    frame #&#8203;52840: 0x0000000100001603 Contents`CaseStrideable.distance(other=a, self=a) at Contents.swift:7:33

In the playground provided, notice that distance(to:) contains this on line 7:

    let indices = [self, other].compactMap(Self.allCases.firstIndex)

Strideable provides a default implementation of == in terms of distance(to:), so when distance(to:) calls allCases.firstIndex(of:), it ends up calling back into distance(to:). Because this default implementation is available, Equatable doesn't derive a == operator as it normally would. (Having this default implementation probably made more sense before Equatable derivation.)

The user can fix this by implementing == by hand. We can't remove this default implementation now that we're ABI-stable; maybe we could deprecate it and make Equatable derivation ignore deprecated == operators or something?

(Edit: Why did JIRA think that expanding smilies in code blocks was a good idea?)

[glessard]

This is similar to https://bugs.swift.org/browse/SR-7117

(There, it's Strideable's default implementation of < that is the issue.)