Open Radar

 
// The following code demonstrates the problem with dynamic member lookup and optimization.
// Without optimization, the following code prints the following:
//
//    ==== Setter for keypath: \TempAPIContainerFields.foo
//    ==== Setter for keypath: \TempAPIContainerFields.bar
//    ==== Getter for keypath: \TempAPIContainerFields.foo.
//    ==== Getter for keypath: \TempAPIContainerFields.bar.
//    \TempAPIContainerFields.foo
//    \TempAPIContainerFields.bar
//
// But when optimization is set to -O2 or higher, accesses to tempAPIContainer.foo and tempAPIContainer.bar are no longer considered to be different,
// and the following output is produced:
//
//    ==== Setter for keypath: \TempAPIContainerFields.bar
//    ==== Setter for keypath: \TempAPIContainerFields.bar
//    ==== Getter for keypath: \TempAPIContainerFields.bar.
//    ==== Getter for keypath: \TempAPIContainerFields.bar.
//    \TempAPIContainerFields.bar
//    \TempAPIContainerFields.bar
//

// My assumption is that dynamic member lookup through .foo and .bar must never resolve to the same keypath,
// however it's the case when optimization is enabled.
// This is broken for both MacOS (15.3.1) and iOS (18+)
// I am using Xcode Version 16.2 (16C5032a)

import Foundation

public struct TempAPIContainerFields {
    public var foo: String.Type = String.self
    public var bar: String.Type = String.self
}

@dynamicMemberLookup public struct TempAPIContainer {
    
    public subscript<T>(dynamicMember keypath: KeyPath<TempAPIContainerFields, T.Type>) -> T {
        get {
            print("==== Getter for keypath: \(keypath).")
            if let castedKeyPath: T = "\(keypath)" as? T {
                return castedKeyPath
            }
            fatalError()
        }
        mutating set { // swiftlint:disable:this unused_setter_value
            print("==== Setter for keypath: \(keypath)")
        }
    }
}

var tempAPIContainer = TempAPIContainer()

let keyPath1 = \TempAPIContainerFields.foo
let keyPath2 = \TempAPIContainerFields.bar

tempAPIContainer.foo = "foo"
tempAPIContainer.bar = "bar"

let foo1 = tempAPIContainer.foo
let bar1 = tempAPIContainer.bar

print(foo1)
print(bar1)