Rapid expansion on a common structure based on type T properties

Question

If I have a general structure, for example ...

struct Blah<T> { let someProperty: T } 

Can I then extend Blah to match Equatable only when T is Equatable . How...

 extension Blah: Equatable where T: Equatable { static func == (lhs: Blah, rhs: Blah) -> Bool { return lhs.someProperty == rhs.someProperty } } 

Is it possible?

I tried several different coding methods, but each of them gives me a slightly different error.

Answer 1

What you are looking for is

• SE-0143 Conditional Compliance

(which, in turn, is part of the "manifest manifest" ). The proposal was accepted for Swift 4, but not yet implemented. From the offer:

Conditional mappings express the idea that a generic type will conform to a particular protocol only when its type arguments satisfy certain requirements.

and a prime example is

 extension Array: Equatable where Element: Equatable { static func ==(lhs: Array<Element>, rhs: Array<Element>) -> Bool { ... } } 

make uniform arrays of equivalent elements, which is impossible at the moment. Your example essentially

 struct SomeWrapper<Wrapped> { let wrapped: Wrapped } extension SomeWrapper: Equatable where Wrapped: Equatable { static func ==(lhs: SomeWrapper<Wrapped>, rhs: SomeWrapper<Wrapper>) -> Bool { return lhs.wrapped == rhs.wrapped } } 

from this sentence.