Proposed resolution:

Remove explicit.

namespace std {
template<class> class function;
// undefined
template<class R, class... ArgTypes>
class function<R(ArgTypes...)>
: public unary_function<T1, R>
// iff sizeof...(ArgTypes) == 1 and ArgTypes contains T1
: public binary_function<T1, T2, R>
// iff sizeof...(ArgTypes) == 2 and ArgTypes contains T1 andT2
{
public:typedef R result_type;
// 20.8.14.2.1, construct/copy/destroy:
  explicit function();

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Addresses JP-3

explicit default contructor is defined in std::function. Although it is allowed according to 12.3.1, it seems unnecessary to qualify the constructor as explicit. If it is explicit, there will be a limitation in initializer_list.