struct Foo {
    struct Bar: Hashable {
        static func ==(lhs: Bar, rhs: Bar) -> Bool { return true }
        var hashValue: Int { return 0 }
    }
}

var barMap: [Foo.Bar: String] = [:] // <-- Works
var barMap1 = [Foo.Bar: String]() // <-- error: type of expression is ambiguous without more context