What do you mean by ‘use them in other USTRUCTs’?
If you mean embedding them like:
USTRUCT()
struct X
{
UPROPERTY()
struct A a;
};
then that isn’t possible in C++ at all. Member variables which aren’t pointers/references need to have their type fully defined.
If that’s what you’re doing, then yeah you’ll get a UHT error, but actually it’s irrelevant, since it couldn’t compile even if the UHT pass succeeded.