...same as aef1826d, actually fallout from
diverging behavior regarding DR 253 betwween GCC/libstdc++ and Clang, cf. thread
starting at <https://gcc.gnu.org/ml/libstdc++/2015-04/msg00165.html>
"std::__debug::vector default ctor must not be defaulted"

Change-Id: I9189d472b57249857d64f7d807a0e08fc1758759