The EphemeralForBtreeImplTestHarnessHelper class builds a SortedDataInterface with an empty index key specification. This leads the BuilderAddSameKey test in the storage_ephemeral_for_test_btree_test suite, which generates a DuplicateKey exception, to call buildDupKeyErrorStatus in index_entry_comparison.cpp with an empty key definition and a non-empty set of key values.

This behavior prevents us from adding stronger invariants to buildDupKeyErrorStatus, such as the following: 

invariant(keyPattern.nFields() == key.nFields()); 

This would be a good addition as this method iterates over both key and values in a single loop, with the assumption that they contain the same number of elements.