refactor: use transparent comparator for setBlockIndexCandidates lookups

This allows checking for existence in setBlockIndexCandidates using a const CBlockIndex* without casting away constness, replacing a legacy const_cast check in validation.cpp.

src/node/blockstorage.h

@@ -136,6 +136,7 @@ using BlockMap = std::unordered_map<uint256, CBlockIndex, BlockHasher>;
 
 struct CBlockIndexWorkComparator {
     bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
+    using is_transparent = void;
 };
 
 struct CBlockIndexHeightOnlyComparator {

src/validation.cpp

@@ -5401,7 +5401,7 @@ void ChainstateManager::CheckBlockIndex() const
                         // needs to be added if it is an ancestor of the target
                         // block.
                         if (!c->TargetBlock() || c->TargetBlock()->GetAncestor(pindex->nHeight) == pindex) {
-                            assert(c->setBlockIndexCandidates.contains(const_cast<CBlockIndex*>(pindex)));
+                            assert(c->setBlockIndexCandidates.contains(pindex));
                         }
                     }
                     // If some parent is missing, then it could be that this block was in
@@ -5409,7 +5409,7 @@ void ChainstateManager::CheckBlockIndex() const
                     // In this case it must be in m_blocks_unlinked -- see test below.
                 }
             } else { // If this block sorts worse than the current tip or some ancestor's block has never been seen, it cannot be in setBlockIndexCandidates.
-                assert(!c->setBlockIndexCandidates.contains(const_cast<CBlockIndex*>(pindex)));
+                assert(!c->setBlockIndexCandidates.contains(pindex));
             }
         }
         // Check whether this block is in m_blocks_unlinked.
@@ -5441,7 +5441,7 @@ void ChainstateManager::CheckBlockIndex() const
             // So if this block is itself better than any m_chain.Tip() and it wasn't in
             // setBlockIndexCandidates, then it must be in m_blocks_unlinked.
             for (const auto& c : m_chainstates) {
-                if (!CBlockIndexWorkComparator()(pindex, c->m_chain.Tip()) && !c->setBlockIndexCandidates.contains(const_cast<CBlockIndex*>(pindex))) {
+                if (!CBlockIndexWorkComparator()(pindex, c->m_chain.Tip()) && !c->setBlockIndexCandidates.contains(pindex)) {
                     if (pindexFirstInvalid == nullptr) {
                         if (!c->TargetBlock() || c->TargetBlock()->GetAncestor(pindex->nHeight) == pindex) {
                             assert(foundInUnlinked);