TreeIter: Donʼt return dangling ref on *rvalueIter
operator*() was always returning a reference, i.e. T&, but it does so by
casting *this to the T type and then returning a reference to that. This
results in a dangling reference if the TreeIter instance was an rvalue –
for instance in auto& row = *model->append(), the function call returns
an rvalue TreeIter, which we merrily let be cast to an lvalue TreeRow&…!
This results in crashes when running such code.
The fix is to prevent that code from compiling, by making the current operator*() only valid for const& i.e. lvalue instances – and add an overload for rvalue instances that returns a T as-is, without reference.
Edited by Daniel Boles