Proposed resolution:

Add explicit.

namespace std {
template <class Key, class Compare = less<Key>,
class Allocator = allocator<Key> >
class set {
public:
...
explicit set(const Allocator&);

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

Addresses JP-8

Constructor accepting an allocator as a single parameter should be qualified as explicit.