GenericProtocol.swift

import Foundation
import XCTest

public protocol A {
    var value: Int { get }
}

public protocol B {
    associatedtype T: A
    func check<T>(input: T) -> Int
}

struct Impl<U>: B where U: A {

    typealias T = U

    let path: KeyPath<U, Int>

    func check<T>(input: T) -> Int {
        return (input as! U)[keyPath: path]
    }
}

final class GenericTests: XCTestCase {

    func testOne() {
        struct One: A {
            var value: Int
        }

        struct Two: A {
            var value: Int
        }

        let impl = Impl(path: \One.value)

        let one = One(value: 1)
        let two = Two(value: 2)

        let _ = impl.check(input: one)  // 1
        let _ = impl.check(input: two)  // error
    }

}