GH-33911: [C++] Add missing std::forward to Result::ValueOrElse (#33912)

It fixes #33911.

------

From #33911:

> https://github.com/apache/arrow/blob/4f1d255f3dc57457e5c78d98c4b76fc523cf9961/cpp/src/arrow/result.h#L369-L375
>
> The parameter `generate_alternative` is passed by univeral forwarding, but `std::forward` is missing to use it.
>
> This may result in incorrect overloading selection or compilation errors, such as a class type with the method `operator()() &&`.
* Closes: #33911

Lead-authored-by: PragmaTwice <twice.mliu@gmail.com>
Co-authored-by: Twice <twice@apache.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/result.h

@@ -371,7 +371,7 @@ class [[nodiscard]] Result : public util::EqualityComparable<Result<T>> {
     if (ok()) {
       return MoveValueUnsafe();
     }
-    return generate_alternative();
+    return std::forward<G>(generate_alternative)();
   }
 
   /// Apply a function to the internally stored value to produce a new result or propagate

cpp/src/arrow/result_test.cc

@@ -28,6 +28,7 @@
 #include "arrow/testing/gtest_compat.h"
 #include "arrow/testing/gtest_util.h"
 #include "arrow/testing/matchers.h"
+#include "arrow/util/functional.h"
 
 namespace arrow {
 
@@ -795,5 +796,14 @@ TEST(ResultTest, MatcherExplanations) {
   }
 }
 
+TEST(ResultTest, ValueOrGeneratedMoveOnlyGenerator) {
+  Result<MoveOnlyDataType> result = Status::Invalid("");
+  internal::FnOnce<MoveOnlyDataType()> alternative_generator = [] {
+    return MoveOnlyDataType{kIntElement};
+  };
+  auto out = std::move(result).ValueOrElse(std::move(alternative_generator));
+  EXPECT_EQ(*out.data, kIntElement);
+}
+
 }  // namespace
 }  // namespace arrow