message: Check for merge of leaf and non-leaf partitions.
  Merge of a non-leaf partition and a leaf partition produces a non-leaf
  partition with token range wider than concatenated token ranges of its
  children.
diff: @@ -296,8 +296,17 @@ void IndentButPreserveOtherSpacing(TokenPartitionRange partition_range,
  }
  void MergeConsecutiveSiblings(TokenPartitionTree* tree, size_t pos) {
  + CHECK_NOTNULL(tree);
  + CHECK_LT(pos + 1, tree->Children().size());
  + const auto& current = tree->Children()[pos];
  + const auto& next = tree->Children()[pos + 1];
  + // Merge of a non-leaf partition and a leaf partition produces a non-leaf
  + // partition with token range wider than concatenated token ranges of its
  + // children.
  + CHECK(current.is_leaf() == next.is_leaf()) << "left:\n"
  + << current << "\nright:" << next;
  // Effectively concatenate unwrapped line ranges of sibling subpartitions.
  - ABSL_DIE_IF_NULL(tree)->MergeConsecutiveSiblings(
  + tree->MergeConsecutiveSiblings(
  pos, [](UnwrappedLine* left, const UnwrappedLine& right) {
  // Verify token range continuity.
  CHECK(left->TokensRange().end() == right.TokensRange().begin());
  