Hi,

I'm wondering why `using namespace` directives are not allowed within
class definitions? It would seem not fundamentally different from other
contexts, like within a function body.

The use case I have in mind is related to user-defined literals:

  struct foo
  {
    using namespace std::literals;

    std::string str;

    foo() : str("abcd"sv) {} // using-directive needed for this
  };

PS: Yes, I know I could move the constructor definition to namespace
scope and use the using directive there. I don't want to, because I want
this constructor to be inline, in the header, and yet I don't want to
have using directives in a public header at the namespace scope.