[Bug libstdc++/48101] obscure error message with std::set<const int>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48101

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Which causes the code to be accepted. I'd rather do:

template<typename T> class allocator<const T>;  // undefined

so there's an error.