[SR-14408] Improve diagnostic when trying to use enum case without payload

[typesanitizer]

Previous ID	SR-14408
Radar	rdar://17718836
Original Reporter	@typesanitizer
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, TypeChecker
Assignee	None
Priority	Medium

md5: 34e58d22ee98f59e77c57a4b3543f250

is duplicated by:

• SR-14428 Comparing enum with associated value gives misleading error about unrelated enum

Issue Description:

enum CompassPoint {
    case North(Int)
    case South
    case East
    case West
}

func isNorth(c : CompassPoint) -> Bool {
  return c == .North
// error: binary operator '==' cannot be applied to operands of type 'CompassPoint' and '_' return c == .North
// error: cannot infer contextual base in reference to member 'North' 
}

func isSouth(c : CompassPoint) -> Bool {
  return c == .South
// error: binary operator '==' cannot be applied to two 'CompassPoint' operands
// note: binary operator '==' cannot be synthesized for enums with associated values 
}

Notice that the diagnostic is better for the c == .South case. It would be nice if the type-checker could recognize that in the c == .North case, the user was trying to access CompassPoint.North and emit a more useful diagnostic.

[typesanitizer]

cc luciano (JIRA User), potentially of interest to you since you often end up tackling these kinds of diagnostic improvements. 🙂

[LucianoPAlmeida]

Interested! Thanks for the ping theindigamer (JIRA User) =]
I think this is somewhat related to SR-12908, not exactly sure is a dupe but I think they are similar

[typesanitizer]

Good spot, they are quite similar. There's a slight difference in that, in this case, CompassPoint doesn't have an Equatable conformance, whereas in the latter case, Result does have a conditional conformance but that is not used because TwoValues does not have an Equatable conformance.

I don't know if that makes this one easier to provide a better diagnostic for, compared to SR-12908.

[typesanitizer]

In the linked SR-14428, the issue is even worse since the compiler gives an actively misleading diagnostic.

[LucianoPAlmeida]

Fixed by #39039