static inline Signature getEmptyKey() { return {}; }
    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();