message: quench warning on Clang 9
diff: @@ -204,7 +204,7 @@ template <typename Value> struct Tape<Value>::SimplificationLock {
  };
  template <typename Value> std::unique_ptr<Tape<Value>> Tape<Value>::s_tape;
  -template <typename Value> Tape<Value> *Tape<Value>::get() {
  +template <typename Value> ENOKI_PURE Tape<Value> *Tape<Value>::get() {
  if (ENOKI_UNLIKELY(!s_tape))
  s_tape = std::unique_ptr<Tape>(new Tape());
  return s_tape.get();
  