For conditionals, we currently work around the lack of this feature by using the ternary operator nested within the QStringBuilder expression:

QStringView foo = ~~~;
QString s = str1 % ~~~~ % (shouldIncludeFoo ? foo : u"") % ~~~~ ;

But this requires the LHS and RHS of the ternary to have a reasonably-identical type. I cannot add a QL1S in one and a QStringView in the other, let alone QStringBuilder<T,S> for differing T, S.

I'm not sure whether one could just overload the ternary operator for QStringBuilder<>, but that would probably mean that at least one of the legs has to be a QStringBuilder expression itself, so maybe something more explicit is needed, like QStringBuilder::Conditional(cond, lhs, rhs)?

For repetition we don't have any work-around, except storing a subexpression in QString and then using QString::repeated().