[SR-9397] Failed to demangle witness for associated type (CF_ENUM)

[swift-ci]

Previous ID	SR-9397
Radar	rdar://46423275
Original Reporter	swhitty (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 5.0Regression, Runtime
Assignee	@DougGregor
Priority	Medium

md5: 964a83a758ac083db82c1cca841cc98f

Issue Description:

The Swift 5 toolchain includes a regression that results in a runtime error;

failed to demangle witness for associated type ....
Abort trap: 6

This occurs when a generic type references a nested associated type that is a CF_ENUM / NS_ENUM from Foundation (or possibly another module??). The issue does not occur within Swift 4.2.

Consider the following;

protocol CommandTypes {
    associatedtype Result
    associatedtype Message
}

struct EnumCommand: CommandTypes {
    typealias Result = ComparisonResult
    typealias Message = String
}

struct Command<T: CommandTypes> {
    var result: T.Result?
    var message: T.Message?
}

❌In Swift 5, declaring an array which references an associated type of CF_ENUM results in abort Trap 6. e.g.

let _ = [Command<EnumCommand>]()

✅Declaring an array with other types is fine;

struct BasicCommand: CommandTypes {
    typealias Result = String
    typealias Message = String
}

let _ = [Command<BasicCommand>]()

✅One workaround is to access the CF_ENUM type at least once prior e.g.

let _ = [ComparisonResult.self] as [Any]
let _ = [Command<EnumCommand>]()

[belkadan]

@mikeash, @DougGregor, @slavapestov, sound familiar?

[slavapestov]

@swift-ci create

[DougGregor]

The full error message is:

failed to demangle witness for associated type 'Result' in conformance 't2.EnumCommand: CommandTypes' from mangled name 'So18NSComparisonResultV' 

We're not emitting type metadata that we need to be able to find NSComparisonResult at run time.

[DougGregor]

Fixed by #21007