-Wambiguous-reversed-operator

[ebraminio]

We get a -Wambiguous-reversed-operator warning in -std=c++2a mode in new clang builds, maybe an interesting thing or maybe not,

clang -c src/harfbuzz.cc src/hb-subset*.cc -DHB_NO_MT -Werror -std=c++2a

After the fix the warning can be removed from https://github.com/harfbuzz/harfbuzz/blob/5ea11cb/.circleci/config.yml#L121

src/hb-vector.hh:256:21: error: ISO C++20 considers use of overloaded operator '==' (with operand types 'hb_user_data_array_t::hb_user_data_item_t' and 'hb_user_data_array_t::hb_user_data_item_t') to be ambiguous despite there being a unique best viable function [-Werror,-Wambiguous-reversed-operator]
      if (arrayZ[i] == v)
          ~~~~~~~~~ ^  ~
src/hb-object.hh:56:26: note: in instantiation of function template specialization 'hb_vector_t<hb_user_data_array_t::hb_user_data_item_t>::find<hb_user_data_array_t::hb_user_data_item_t>' requested here
    item_t *item = items.find (v);
                         ^
src/hb-static.cc:98:22: note: in instantiation of function template specialization 'hb_lockable_set_t<hb_user_data_array_t::hb_user_data_item_t, hb_mutex_t>::replace_or_insert<hb_user_data_array_t::hb_user_data_item_t>' requested here
  bool ret = !!items.replace_or_insert (item, lock, (bool) replace);
                     ^
src/hb-object.hh:172:10: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
    bool operator == (hb_user_data_item_t &other) const { return key == other.key; }
         ^
1 error generated.