I thought that without the need of using TypeCombinator::union and just a new UnionType instead we could also avoid a dedicated property (cause there no need to avoid this constructor call isn't it) and save some memory (even if I assume it's small).

new UnionType([...]); creates a non-normalized union type, which means later calls to TypeCombinator::* need to normalize it.

TypeCombinator::union() returns a normalized union type, which means we also move this repeated cost of normalization to the construction time and do it only once instead of every time.

Cause if the goal is even to save new *Type calls, we'll end with private property everywhere...

I wouldn't say the goal is to save all the new *Type calls. this change was motivated by a blackfire profile which showed produceArrayDimFetchAssignValueToWrite is the slowest consumer of TypeCombinator::intersect().

the change was done to get rid of a hot path. it does not mean similar pattern in other less-hot path should be treated the same.