public protocol P {}

public struct S: Differentiable {
  var x: Float = 0
  public init() {}

  //public struct TangentVector: AdditiveArithmetic, Differentiable {
  //  var x: Float = 0
  //}
  //public func move(along direction: TangentVector) { fatalError() }
}

extension S.TangentVector: P {}

func f<T: Differentiable>(_ t: T) where T.TangentVector: P {}

f(S())

module.swift:17:1: error: global function 'f' requires that 'S.TangentVector' conform to 'P'
f(S())
^
module.swift:15:6: note: where 'T.TangentVector' = 'S.TangentVector'
func f<T: Differentiable>(_ t: T) where T.TangentVector: P {}
     ^