Pavel Yaskevich cde23eccc8 [ConstraintSystem] Implement keypath dynamic member lookup ...

Implement keypath based dynamic member lookup which augments
functionality of existing @dynamicMemberLookup attribute.

Two new subscript overloads are accepted:

```
subscript(dynamicMember member: KeyPath<T, ...>) -> ...
subscript(dynamicmember member: WritableKeyPath<T, ...>) -> ...
```

Example:

```swift
struct Point {
  let x: Int
  var y: Int
}

@dynamicMemberLookup struct Lens<T> {
  var obj: T

  init(_ obj: T) {
    self.obj = obj
  }

  subscript<U>(dynamicMember member: KeyPath<T, U>) -> Lens<U> {
    get { return Lens<U>(obj[keyPath: member]) }
  }

  subscript<U>(dynamicMember member: WritableKeyPath<T, U>) -> Lens<U> {
    get { return Lens<U>(obj[keyPath: member]) }
    set { obj[keyPath: member] = newValue.obj }
  }
}

var lens = Lens(Point(x: 0, y: 0))

_ = lens.x // converted into `lens[dynamicMember: KeyPath<Point, Int>`
_ = lens.y = Lens(10) // converted into `lens[dynamicMember: WritableKeyPath<Point, Int>]`
```

2019-04-01 12:40:39 -07:00

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