Value of protocol type 'Encodable' cannot conform to 'Encodable'; only struct/enum/class types can conform to protocols

Encodable cannot be used as an annotated type. It can be only used as a generic constraint. And JSONEncoder can encode only concrete types.

The function

func doStuff<T: Encodable>(payload: [String: T]) {

is correct but you cannot call the function with [String: Encodable] because a protocol cannot conform to itself. That's exactly what the error message says.

---

The main problem is that the real type of things is [String:Any] and Any cannot be encoded.

You have to serialize things with JSONSerialization or create a helper struct.

You're trying to conform T to Encodable which is not possible if T == Encodable. A protocol does not conform to itself.

Instead you can try:

func doStuff<T: Hashable>(with items: [T: Encodable]) {
    ...
}