Swift – Member not found when accessing a value through two conditional subscripts

swift-member-not-found-through-two-conditional-subscripts.swift

// MARK: - CRS

protocol CoordinateReferenceSystem {
	associatedtype CoordinateComponents
}

protocol TwoDimensionsCRS: CoordinateReferenceSystem where CoordinateComponents == (X, Y) {
	associatedtype X
	associatedtype Y
}

protocol GeographicCRS: CoordinateReferenceSystem {}
protocol GeocentricCRS: CoordinateReferenceSystem {}

enum WGS84Geographic2DCRS: TwoDimensionsCRS, GeographicCRS {
	typealias X = Double
	typealias Y = Double
}

// MARK: - Coordinates

@dynamicMemberLookup
protocol Coordinates<CRS> {
	associatedtype CRS: CoordinateReferenceSystem
	typealias Components = CRS.CoordinateComponents
	var components: Components { get }
	subscript<T>(dynamicMember keyPath: KeyPath<Components, T>) -> T { get }
}

extension Coordinates {
	subscript<T>(dynamicMember keyPath: KeyPath<Components, T>) -> T {
		self.components[keyPath: keyPath]
	}
}

protocol TwoDimensionsCoordinates: Coordinates where CRS: TwoDimensionsCRS {
	var x: CRS.X { get }
	var y: CRS.Y { get }
}

extension TwoDimensionsCoordinates {
	var x: CRS.X { self.components.0 }
	var y: CRS.Y { self.components.1 }
}

extension TwoDimensionsCoordinates where CRS: GeographicCRS {
	var latitude: CRS.X { self.x }
	var longitude: CRS.Y { self.y }
}

struct Coordinates2D: TwoDimensionsCoordinates {
	typealias CRS = WGS84Geographic2DCRS
	var components: Components
}

// MARK: - Points

@dynamicMemberLookup
protocol Point<C, Metadata> {
	associatedtype C: Coordinates
	associatedtype Metadata
	var coordinates: C { get }
	var metadata: Metadata { get }
	subscript<T>(dynamicMember keyPath: KeyPath<C, T>) -> T { get }
}

extension Point {
	subscript<T>(dynamicMember keyPath: KeyPath<C, T>) -> T {
		self.coordinates[keyPath: keyPath]
	}
}

// Doesn't change anything
//extension Point where C.CRS: GeographicCRS {
//	subscript<T>(dynamicMember keyPath: KeyPath<C, T>) -> T {
//		self.coordinates[keyPath: keyPath]
//	}
//}

struct Point2D {
	typealias Metadata = Void
	var coordinates: Coordinates2D
	var metadata: Metadata
}

// MARK: - Code playground

let point = Point2D(coordinates: .init(components: (2, 3)), metadata: ())
let l1 = point.coordinates.latitude
// error: value of type 'Point2D' has no member 'latitude'
let l2 = point.latitude

Posted on https://forums.swift.org/t/member-not-found-when-accessing-a-value-through-two-conditional-subscripts/61017