[SR-15467] Confusing error message for unavailable member when conforming to protocol #57772
Description
| Previous ID | SR-15467 |
| Radar | None |
| Original Reporter | vinivendra (JIRA User) |
| Type | Improvement |
Attachment: Download
Environment
MacOS 12.0.1 - Swift 5.5.1
Additional Detail from JIRA
| Votes | 0 |
| Component/s | Compiler |
| Labels | Improvement |
| Assignee | None |
| Priority | Medium |
md5: b7664ab271752893116e32098a0c909a
relates to:
- SR-15466 Duplicated error message for type that doesn't conform to protocol
Issue Description:
When creating a type that conforms to Collection, create implementations for all requirements except for `subscript(bounds: Range<Index>) -> Subsequence`. This will cause the compiler to try to satisfy this requirement with an unavailable subscript, which will raise the following error:
test.swift:2:15: error: type 'ListSlice<Element>' does not conform to protocol 'Collection'
public struct ListSlice<Element>: Collection {
^
test.swift:2:15: error: unavailable subscript 'subscript(_:)' was used to satisfy a requirement of protocol 'Collection'
public struct ListSlice<Element>: Collection {
^
Swift.Collection:3:12: note: 'subscript(_:)' declared here
public subscript(bounds: Range<Self.Index>) -> Self.SubSequence { get }
^
Swift.Collection:12:5: note: requirement 'subscript(_:)' declared here
subscript(bounds: Range<Self.Index>) -> Self.SubSequence { get }This error has the following problems:
-
It is unlikely that the user knows about this default implementation that is no longer available, and the error doesn't point to that implementation (which, in this case, contains a comment describing why it's no longer available).
-
Saying the unavailable subscript "was used to satisfy a requirement" makes it seem as if a user's subscript was used to satisfy the requirement, when in fact this refers to a subscript in the compiler (that the user likely doesn't know about).
-
The notes don't show up in Xcode, and by only reading the error message it's unclear which required subscript is the problem. For instance, Collection requires implementing both `subscript(position: Int) -> Element` and `subscript(bounds: Range<Index>) -> SubSequence`, both of which match the 'subscript(_🙂' signature in the error message.
This error could be improved if it read something like: “*default implementation of 'Collection' requirement 'subscript(*🙂' of type '(Range<Self.Index>) -> Self.SubSequence' is marked unavailable; did you mean to implement your own?_”. This message makes it clear that the error refers to a default implementation; that the problem is in the "Range" subscript; and that the user can implement their own subscript to solve it.
It would also be useful to add a fix-it to help the user add their own implementation.
The attached test.swift file causes these errors in Swift 5.5.1.