Use macro instead of [[maybe_unsued]]

Use macro that works both for clang and g++ instead of [[maybe_unused]]
(because g++ produces a warning when [[maybe_unsued]] is present,
and clang -- when isn't)

src/core/algorithms/fd/pyrocommon/model/pli_cache.h

@@ -8,6 +8,7 @@ class ProfilingContext;
 #include "cache_eviction_method.h"
 #include "caching_method.h"
 #include "model/table/column_layout_relation_data.h"
+#include "util/maybe_unused.h"
 
 namespace model {
 
@@ -29,20 +30,20 @@ class PLICache {
     std::unique_ptr<VerticalMap<PositionListIndex>> index_;
     // usageCounter - for parallelism
 
-    // int saved_intersections_ = 0;
+    MAYBE_UNUSED int saved_intersections_ = 0;
 
     mutable std::mutex getting_pli_mutex_;
 
     CachingMethod caching_method_;
-    [[maybe_unused]] CacheEvictionMethod eviction_method_;
-    [[maybe_unused]] double caching_method_value_;
-    [[maybe_unused]] long long maximumAvailableMemory_ = 0;
+    MAYBE_UNUSED CacheEvictionMethod eviction_method_;
+    MAYBE_UNUSED double caching_method_value_;
+    // long long maximumAvailableMemory_ = 0;
     double maximum_entropy_;
-    [[maybe_unused]] double mean_entropy_;
-    [[maybe_unused]] double min_entropy_;
-    [[maybe_unused]] double median_entropy_;
-    [[maybe_unused]] double median_gini_;
-    [[maybe_unused]] double median_inverted_entropy_;
+    MAYBE_UNUSED double mean_entropy_;
+    MAYBE_UNUSED double min_entropy_;
+    MAYBE_UNUSED double median_entropy_;
+    MAYBE_UNUSED double median_gini_;
+    MAYBE_UNUSED double median_inverted_entropy_;
 
     std::variant<PositionListIndex*, std::unique_ptr<PositionListIndex>> CachingProcess(
             Vertical const& vertical, std::unique_ptr<PositionListIndex> pli,

src/core/model/table/vertical_map.cpp

@@ -8,6 +8,7 @@
 #include "fd/pyrocommon/core/vertical_info.h"
 #include "fd/pyrocommon/model/agree_set_sample.h"
 #include "position_list_index.h"
+#include "util/maybe_unused.h"
 
 namespace model {
 
@@ -425,7 +426,7 @@ void VerticalMap<Value>::Shrink(double factor, std::function<bool(Entry, Entry)>
             key_queue.push(entry);
         }
     });
-    [[maybe_unused]] unsigned int num_of_removed = 0;
+    MAYBE_UNUSED unsigned int num_of_removed = 0;
     unsigned int target_size = size_ * factor;
     while (!key_queue.empty() && size_ > target_size) {
         auto key = key_queue.top().first;
@@ -467,7 +468,7 @@ void VerticalMap<Value>::Shrink(std::unordered_map<Vertical, unsigned int>& usag
                     key_queue.push(entry);
                 }
             });
-    [[maybe_unused]] unsigned int num_of_removed = 0;
+    MAYBE_UNUSED unsigned int num_of_removed = 0;
     while (!key_queue.empty()) {
         auto key = key_queue.front().first;
         key_queue.pop();

src/core/util/maybe_unused.h

@@ -0,0 +1,10 @@
+#pragma once
+
+// clang produces warning on unused private fields, so they need to be marked as [[maybe_unused]],
+// but g++ doesn't recognize [[maybe_unused]] on class fields and produces warning.
+// This macro expands to [[maybe_unsued]], when compiler is clang, nop otherwise
+#ifdef __clang__
+#define MAYBE_UNUSED [[maybe_unused]]
+#else
+#define MAYBE_UNUSED /* Ignore */
+#endif