VERIFY_RESULT creates undesirable copy of Result<T&>'s value

[d-uspenskiy]

VERIFY_RESULT and similar macros uses GNU expression statement __extension__

#define VERIFY_RESULT(expr) \
  __extension__ ({ auto&& __result = (expr); RETURN_NOT_OK(__result); std::move(*__result); })

In spite of the fact last statement is a rvalue copy object is created by using of T(T&&) move-constructor (if any) for Result<T>/Result<T&> situation and T::T(const T&) copy-constructor (if any) for Result<const T&>

This behavior produces several undesirable side effects of using VERIFY_RESULT:

• can't be used for functions/methods which returns Result with non-copyable T even in Result<T&>/Result<const T&> form
• In case T is moveable returning of Result<T&> from function/method original object of T will be set to unspecified state

Result<vector<int>&> GetData() {
   return data_;
} 

simple calling of VERIFY_RESULT(obj.GetData()) will set obj's data_ field to unspecified state (state of moved from object)

• In case T is copyable but not moveable returning of Result<T&>/Result<const T&> creates undesirable copy of T

Result<const map<string, string>&> GetLargeData() const {
  return data_;
}

each call of VERIFY_RESULT(obj.GetLargeData()) will create map's copy