Is it ready that StrideTo and StrideThrough structs conform to RandomAccessCollection?

SusanCheng · 2020-03-19T06:07:49.673Z

The FIXME say that was due to the bug of SR-6474

apple/swift/blob/da61cc8cdf7aa2bfb3ab03200c52c4d371dc6751/stdlib/public/core/Stride.swift#L309


      
              return nil
            }
          }
          
          
extension StrideTo: CustomReflectable {
            public var customMirror: Mirror {
              return Mirror(self, children: ["from": _start, "to": _end, "by": _stride])
            }
          }
          
          
// FIXME(conditional-conformances): This does not yet compile (SR-6474).
          #if false
          extension StrideTo: RandomAccessCollection
          where Element.Stride: BinaryInteger {
            public typealias Index = Int
            public typealias SubSequence = Slice<StrideTo<Element>>
            public typealias Indices = Range<Int>
          
          
  @inlinable
            public var startIndex: Index { return 0 }

and the bug was fixed in 2018.

github.com/apple/swift

[SR-6474] Conditional conformance to refined protocol inappropriately fails

opened 05:28PM - 25 Nov 17 UTC

closed 05:08AM - 21 Mar 21 UTC

xwu

bug Compiler ConditionalConformance

| | | |------------------|-----------------|… |Previous ID | SR-6474 | |Radar | rdar://problem/38754177 | |Original Reporter | @xwu | |Type | Bug | |Status | Closed | |Resolution | Done | <details> <summary>Environment</summary> Swift snapshot 2017-11-24 (a) </details> <details> <summary>Additional Detail from JIRA</summary> | | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Bug, ConditionalConformance | |Assignee | @DougGregor | |Priority | Medium | md5: 831a0c9866177c91e932d3d199ddbb0c </details> **duplicates**: * [SR-7327](https://bugs.swift.org/browse/SR-7327) Conditional conformance does not conform to inherited protocol **is duplicated by**: * [SR-6914](https://bugs.swift.org/browse/SR-6914) Extensions don't add conformance to inherited protocols * [SR-7253](https://bugs.swift.org/browse/SR-7253) Conditional conformance to P fails if P inherits from two other protocols, but not for eg Array **relates to**: * [SR-6840](https://bugs.swift.org/browse/SR-6840) Confusing error in conditional conformance * [SR-6650](https://bugs.swift.org/browse/SR-6650) Conditional conformance not identifying protocol extension implementation * [SR-6922](https://bugs.swift.org/browse/SR-6922) [SE-0143] [release mode] type ... does not conform to protocol ... **Issue Description:** The following code fails to compile with the 2017-11-24 snapshot. ``` java protocol P { var count: Int { get } } protocol Q : P { } // --- struct S<T> { } extension S : Q where T : Equatable { var count: Int { return 1 } } // --- func f<T : P>(_ p: T) { print(p.count) } f(S<Int>()) ``` Changing `extension S : Q where...` to `extension S : P where...` allows the code to compile, but of course `S : Q` should imply `S : P`. Meanwhile, that workaround never permits `S` to conform to `Q`. That is, the following fails to compile: ``` java struct S<T> { } extension S : P where T : Equatable { var count: Int { return 1 } } extension S : Q where T : Equatable { } ``` (The practical upshot of this is that `StrideTo` cannot conditionally conform to `RandomAccessCollection` because that protocol exists in a hierarchy refining other protocols.)

glessard · 2020-03-19T10:24:00.928Z

If it works, which you could presumably find out by building a toolchain, it would be a change in api which would presumably require an evolution proposal.