Future initializer that can accept throwing expressions.

future_init.swift

import Combine
import Foundation

extension Future where Failure == Error {
    convenience init(promise: @escaping (@escaping (Result<Output, Failure>) -> Void) throws -> Void) {
        self.init({ (innerPromise) in
            do {
                try promise { innerPromise($0) }
            } catch {
                innerPromise(.failure(error))
            }
        })
    }
}

var tokens = Set<AnyCancellable>()

Future<User, Error> { (promise) in // Error here if Future<User, Error> is changed to Future<User, Never>
    let user = try JSONDecoder().decode(User.self, from: Data())
    promise(.success(user))
  }.sink(receiveCompletion: { 
      print($0)
  }, receiveValue: {
      print($0)
}).store(in: &tokens)