[swift-users] "Unavailable instance method... used to satisfy a requirement of protocol"

James Dempsey dempsey at mac.com
Fri Sep 15 11:40:20 CDT 2017

I am updating from Xcode 8 to Xcode 9, still using Swift 3.

In my project I am getting warnings of the pattern:

Unavailable instance method ‘compareQueryResult(_:toQueryResult:)’ was used to satisfy a requirement of protocol ‘ResultComparable’.

My project uses an Obj-C++ framework to wrap C++ and expose it in Obj-C.

The instance method in question is declared and implemented in a number of Obj-C classes in this framework.

@interface DocumentQuery: Query {
   - (ComparisonResult)compareQueryResult:(DocumentResult *)result1 toQueryResult:(DocumentResult *)result2;

@interface GroupQuery: Query {
   - (ComparisonResult)compareQueryResult:(GroupResult *)result1 toQueryResult:(GroupResult *)result2;

Another framework imports the Obj-C++ framework. In this framework the protocol is defined in Swift with an associated type:

public protocol ResultComparable
    associatedtype ResultType
    func compareQueryResult(_ r1: ResultType, toQueryResult r2: ResultType) -> ComparisonResult

And conformance is declared:

extension DocumentQuery: ResultComparable {}
extension GroupQuery: ResultComparable {}

This compiled without warnings in Xcode 8 / Swift 3, but they appear in Xcode 9 / Swift 3. Any help in understanding the issue and how to fix it would be greatly appreciated.


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