Here deduction was failing because when we tried to substitute {int,int} 
for Args in decltype(T(std::declval<Args>()...)) we still had Args 
marked as an incomplete/deducible parameter pack, so the expansion 
wasn't fully resolved.  So now we clear the incomplete flag before 
trying to substitute into later default arguments.

Tested x86_64-pc-linux-gnu, applying to trunk.