[SR-14951] diag::witness_unavailable should include the message string from the @available attribute if present #57293

beccadax · 2021-07-21T23:25:16Z


Previous ID	SR-14951
Radar	None
Original Reporter	@beccadax
Type	Improvement
Status	Closed
Resolution	Done

Attachment: Download

Additional Detail from JIRA


Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, StarterBug
Assignee	Rajagopalan (JIRA)
Priority	Medium

md5: 847db0d4ebc811926f2c7650cf658dc0

Issue Description:

Given this code:

protocol P {
  associatedtype T
  func req()
}
extension P {
  func req() {}
}
extension P where T == Self {
  @available(*, unavailable, message: "write it yourself") func req() {}
}

struct NonSelfT: P {
  typealias T = Int
}
struct SelfT: P {
  typealias T = SelfT
}

We produce this output:

<stdin>:15:8: error: type 'SelfT' does not conform to protocol 'P'
struct SelfT: P {
       ^
<stdin>:15:8: error: unavailable instance method 'req()' was used to satisfy a requirement of protocol 'P'
struct SelfT: P {
       ^
<stdin>:9:65: note: 'req()' declared here
  @available(*, unavailable, message: "write it yourself") func req() {}
                                                                ^
<stdin>:3:8: note: requirement 'req()' declared here
  func req()
       ^

Note that “write it yourself” does not appear in any of the diagnostics, and we wouldn’t see it at all if @available hadn’t been on the same line. It should probably appear in the second diagnostic, perhaps like this:

<stdin>:15:8: error: unavailable instance method 'req()' was used to satisfy a requirement of protocol ‘P’: write it yourself

The text was updated successfully, but these errors were encountered:

beccadax · 2021-07-21T23:40:47Z

Tips for a new contributor:

The diagnostic in question is called witness_unavailable; you can see where it's defined and emitted by searching the Swift repository for that string.

To understand what you might need to do, search the code base for availability_decl_unavailable, the diagnostic we use for @available(*, unavailable) in most other scenarios, and look at we're handling the Message member of AvailableAttr. The use site in ExprAvailabilityWalker::diagnoseMemoryLayoutMigration() is particularly straightforward in this respect—the others have complications you won't need to worry about.

If you're new to Swift, the documentation in docs/Diagnostics.md will give you some useful background on how to specify diagnostic messages.

swift-ci · 2021-07-23T19:20:58Z