[SR-3092] Function-level nested types cannot conform to Equatable

[swift-ci]

Previous ID	SR-3092
Radar	rdar://problem/44479781
Original Reporter	ellneal (JIRA User)
Type	Bug
Status	Reopened
Resolution

Environment

Swift build number: Xcode 8.1 (8B62) embedded version
Target OS: iOS
Build OS: macOS Sierra 16A323
Build Tool: Xcode Version 8.1 (8B62)

Additional Detail from JIRA

Votes	5
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: f04b40d7c56cb16b9b79a99c8f79b080

is duplicated by:

• SR-3875 Can't conform to Equatable when type is inside a function
• SR-5939 Type nested in function cannot satisfy operator protocol requirement
• SR-6526 Can't conform a type declared inside a function to Equatable
• SR-8696 Types nested in functions and their operator functions.
• SR-9956 classes defined inside functions can't be made to conform to Equatable?
• SR-11482 Some protocols cannot be conformed by code-block-local types
• SR-13398 Nested Equatable synthesis cannot be overidden.

relates to:

• SR-14795 Unable to conform to Comparable protocol when nested
• SR-8696 Types nested in functions and their operator functions.

Issue Description:

The compiler fails to compile a struct that conforms to Equatable inside a function.

struct RootLevelStruct: Equatable { // compiles fine
    static func ==(lhs: RootLevelStruct, rhs: RootLevelStruct) -> Bool {
        return false
    }
}

class MyClass {
    struct ClassLevelStruct: Equatable { // compiles fine
        static func ==(lhs: ClassLevelStruct, rhs: ClassLevelStruct) -> Bool {
            return false
        }
    }
    
    func someFunction() {
        struct FunctionLevelStruct: Equatable { // Error: Protocol 'Equatable' can only be used a generic constraint because it has Self or associated type requirements
            static func ==(lhs: FunctionLevelStruct, rhs: FunctionLevelStruct) -> Bool {
                return false
            }
        }
    }
}

[belkadan]

Even after this is fixed I'm not sure the compiler would find the == definition correctly, so there may be two issues here.

[swift-ci]

Comment by Ell Neal (JIRA)

I didn't mention it in the issue but the compiler is also giving a "Type 'FunctionLevelStruct' does not conform to protocol 'Equatable'", so I think you're right.

[lilyball]

This issue is very annoying when writing tests for Decoder-related functionality, because I can't use custom types in each function (because I need equality in order to use XCTAssertEqual).

[swift-ci]

Comment by Tim (JIRA)

This seems to be resolved on the Swift 4.2 toolchain.

[lilyball]

After testing, it seems this was fixed in Swift 4.1.

[lilyball]

This isn't fixed. It appeared to go away with Swift 4.1 because of compiler-synthesized Equatable conformance. It appears the synthesized conformance works, but manual conformance doesn't. Tweaking the struct to not be compatible with synthesized conformance demonstrates that it's still broken in Swift 4.2.

func foo() {
    struct Bar: Equatable {
        let x: ()
        static func ==(lhs: Bar, rhs: Bar) -> Bool {
            return true
        }
    }
}

[belkadan]

>< I feel silly that none of us caught that. Thanks, Lily. Opened a new Radar.