static inline Signature getTombstoneKey() { return {nullptr}; }
    static unsigned getHashValue(Signature val) {
      return llvm::hash_combine_range(val.begin(), val.end());
    }
    static bool isEqual(Signature LHS, Signature RHS) {
      if (RHS == getEmptyKey()) return LHS == getEmptyKey();
      if (RHS == getTombstoneKey()) return LHS == getTombstoneKey();
      if (LHS.size() != RHS.size()) return false;