testArray!(op)(a, b) is equivalent to testArray!(op)(cast(Const!(void)[])a, cast(Const!(void)[])b), which allows for testArray!(op)(a, null) avoiding D2 trouble with typeof(null).
See Implementation
testArray!(op)(a, b) is equivalent to testArray!(op)(cast(Const!(void)[])a, cast(Const!(void)[])b), which allows for testArray!(op)(a, null) avoiding D2 trouble with typeof(null).