GCC std::remove_if overwrites removed elements using:
*dstiter = move(*srciter);
This is fine for normal containers, but produces incorrect results when
*dstiter returns a proxy object instead of a reference. In that case,
the proxy object is move-assigned from the source, then goes out of
scope. If the move assignment did not write to underlying storage, as
valptridx proxy objects do not, then incorrect results occur. This
broke ActiveDoor handling (fixed in 4a01fab66d98[1]) and has been a trap
waiting to recur. Apply reference-qualifiers to valptridx objects so
that move-assignment requires an lvalue for the left-hand side. This
permits normal use of move-assignment, but forces a compile error if
std::remove_if or similar are used on valptridx proxy objects.
[1]: 4a01fab66d
