public enum FunctionInModule1_InternalDerivatives {
  public static func f(_ x: Float) -> Float { x }

  @derivative(of: f)
  static func df(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
    (x, { $0 })
  }
  public static func gradFFromModule1(_ x: Float) -> Float { gradient(at: x, in: f) }
}

workspace/ufa.swift:8:82: error: function is not differentiable
  public static func gradFFromModule1(_ x: Float) -> Float { gradient(at: x, in: f) }
                                                                                 ^
workspace/ufa.swift:2:22: note: differentiated functions in '@inlinable' functions must be marked '@differentiable' or have a public '@derivative'
  public static func f(_ x: Float) -> Float { x }
                     ^