The title is confusing, let me clarify by example:
We have this protocol with a mutating method:
protocol P { mutating func f(_ x: Int) -> Int }
And a conforming class (which has to conform with a non-mutating method):
class C: P { func f(_ x: Int) -> Int { return x } }
An instance of this class can be used with a let constant:
let c = C()
c.f(1) // OK
If we make it an existential object conforming to P, the immutability of the
method will be erased:
let c: AnyObject & P = C()
c.f(1) // Cannot use mutating member on immutable value: 'c' is a 'let' constant
A generic context has the same issue:
func f<T: AnyObject & P>(_ arg: T)-> Int { return arg.f(1) } // Cannot use
mutating member on immutable value: ‘arg' is a 'let' constant
My question:
Is it too much work to preserve method non-mutability in in these cases?
The workaround I am using is this:
protocol Q: class, P { func f(_ x: Int) -> Int } // 'Refine' it to be
non-mutating.
extension C: Q {}
// Now these work:
let c: Q = C()
c.f(1) // OK
func f<T: Q>(_ arg: T)-> Int { return arg.f(1) } // OK
This workaround creates a lot of duplication and is hard to maintain. It is not
something that I do often, but when I do, it is pretty annoying.
Supplemental questions:
Have you guys ever ran into this?
Is there already a bug tracking this?
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