In the (only) implementation of XCTAssertUnorderedEqualSequences (where elements are Equatable), it converts the sequence into an Array. Then it iterates through the second sequence, calling firstIndex(of:) on the first array (O(n)), which totals to O(n²). If a Set were used (when the elements are Hashable), then getting the index of an element is O(1), which would make the total algorithmic complexity O(n).

Expected behavior

XCTAssertUnorderedEqualSequences is O(n) when Element: Hashable

Actual behavior

XCTAssertUnorderedEqualSequences is O(n²) when Element: Hashable