MaybeEquatable2.swift

// see also https://gist.github.com/tarunon/85bdeccf8bc8f44b7b27d4974e0a843f

// user interface
public protocol MaybeEquatable {
    func eraseToOpaqueEquatable() -> ErasedOpaqueEquatable
}

extension MaybeEquatable {
    public func isEqual(to other: Any) -> Bool {
        eraseToOpaqueEquatable().isEqual(to: other)
    }
}

extension MaybeEquatable {
    public func eraseToOpaqueEquatable() -> ErasedOpaqueEquatable {
        ErasedOpaqueEquatable()
    }
}

extension MaybeEquatable where Self: Equatable {
    public func eraseToOpaqueEquatable() -> ErasedOpaqueEquatable {
        TypedOpaqueEquatable(self)
    }
}

public class ErasedOpaqueEquatable {
    internal func isEqual(to other: Any) -> Bool {
        false
    }
}

internal final class TypedOpaqueEquatable<T: Equatable>: ErasedOpaqueEquatable {
    private var left: T
    
    internal init(_ left: T) {
        self.left = left
    }
    
    internal override func isEqual(to right: Any) -> Bool {
        guard let right = right as? T else { return false }
        return left == right
    }
}

// end user interface
func anyIsEqual(_ a: Any, _ b: Any) -> Bool {
    guard let a = a as? MaybeEquatable else { return false }
    return a.isEqual(to: b)
}

// you need to mark your type
extension String: MaybeEquatable {}

print(anyIsEqual("str", "str")) // => true
print(anyIsEqual({}, {})) // => false