This is not needed because keyTypes is never modified directly inside the class. E.g. setOffsetValueType is using ConstantArrayTypeBuilder, which creates a new ConstantArrayType, which unions the keys again before passing them to the ArrayType parent. Meaning, parent::getKeyType is always save to use and contains the latest union already.

So this saves another union operation there and should, in theory, improve performance. But the problematic scenarios that I have lying around seem to be not affected by it and therefore I can't deliver a good-looking before/after comparison. But anyways, unnecessary code is unnecessary code :)