Is there any way to fetch the original decoder for a KeyedDecodingContainer?

Evolution Discussion
4

There's no safe way to simply cast between two unrelated types that I'm aware of, and Container<A> and Container<B> are unrelated alright.

It's also worth mentioning that you can define a custom decode(_:forKey:) function which will be picked up by the synthesizer. It should give you enough leeway to ignore/redirect the key. This technique can be gleaned from ResilientDecoding. Note that the wrapper still needs to conform to Decoder. As an example, the following code uses our custom decode function:

import Foundation 

@propertyWrapper
struct TestWrap<T: ExpressibleByIntegerLiteral>: Decodable {
    var wrappedValue: T
    
    init(from decoder: Decoder) throws { fatalError() }
    init() { wrappedValue = 3 }
}
extension KeyedDecodingContainer {
    func decode<T>(_: TestWrap<T>.Type, forKey: Key) -> TestWrap<T> {
        .init()
    }
}
struct Test: Decodable {
    @TestWrap var a: Int
}

let raw = #"{}"#, decoder = JSONDecoder()
let result = try decoder.decode(Test.self, from: .init(raw.utf8))
print(result.a) // 3

Try not to use this trick too much, though. It can be tricky to reason with.