struct Thing<Foo, Bar> {
    func methodOnAllThings() {}
}

extension Thing where Foo == Int {
    func methodOnThingsWhereFooIsInt() {}
}

extension Thing where Foo == Bool {
    func methodOnThingsWhereFooIsBool() {}
}

func test() {
    
    let thing = Thing<Int, Double>()
    
    thing.methodOnThingsWhereFooIsBool() // ERROR: Generic parameter 'Bar'
                                         //        could not be inferred
    
}
test()