Swift 4.2 regression

gistfile1.swift

import Foundation

struct AnyError: Error {

  private let error: Error

  init(_ error: Error) {
    self.error = error
  }

}

enum Result<T, E: Error> {

  case success(T)
  case failure(E)

  func mapError<NewError: Error>(_ transform: (E) -> NewError) -> Result<T, NewError> {
    switch self {
    case .success(let value):
      return .success(value)
    case .failure(let error):
      return .failure(transform(error))
    }
  }

}

protocol Service {

  func execute<T>() -> Result<T, AnyError>

}

struct Foo {

  enum Bar: Error {

    case foo
    case bar(Error)

  }

}

final class Baz {

  func someFunction<T>(service: Service) -> Result<T, Foo.Bar> {
    return service.execute().mapError(Foo.Bar.bar)
  }

}