1

Why can't I change the properties of a returned generic value?

Take a look at AnimationProvider fails().

Strangely the code isn't failing when I set the returned value to a variable/constant and change the properties of the variable/constant. See AnimationProvider works().

How can I change the code, so that I can change the returned value's properties directly?

protocol AnimatableProperty { }
extension CGFloat: AnimatableProperty { }

protocol Animation<Value>: AnyObject {
    associatedtype Value: AnimatableProperty
    var value: Value { get set }
    var target: Value { get set }
}

class SpringAnimation<Value: AnimatableProperty>: Animation {
    var value: Value
    var target: Value
    init(value: Value, target: Value) {
        self.value = value
        self.target = target
    }
}

class AnimationProvider {
    var animations: [PartialKeyPath<AnimationProvider>: any Animation] = [:]
        
    func animation<Value: AnimatableProperty>(for keyPath: KeyPath<AnimationProvider, Value>) -> (any Animation<Value>)? {
        animations[keyPath] as? SpringAnimation<Value>
    }
    
    var animatableProperty: CGFloat = 4
    
    func crashesCompiler() {
        animation(for: \.animatableProperty)?.value = 6.0
    }
    
    func compiles() {
        let animation = animation(for: \.animatableProperty)
        animation?.value = 6.0
    }
}
2

Looks like a compiler bug.

Smaller demo:

protocol P<Value>: AnyObject {
  associatedtype Value: SignedInteger
  var value: Value { get set }
}

func foo<T: SignedInteger>(_: T) -> (any P<T>)? {
  fatalError()
}
  
func crashesCompiler() {
  foo(123)?.value = 1
}
  
func compiles() {
  let x = foo(123)
  x?.value = 1
}

Note that if we change foo to take the type rather than a value of the type, it works:

protocol P<Value>: AnyObject {
  associatedtype Value: SignedInteger
  var value: Value { get set }
}

func foo<T: SignedInteger>(_: T.Type) -> (any P<T>)? {
  fatalError()
}
  
func noLongerCrashesCompiler() {
  foo(Int.self)?.value = 1
}
  
func compiles() {
  let x = foo(Int.self)
  x?.value = 1
}