c++ - Variable template at class scope

Question

Using N3651 as a basis,

A variable template at class scope is a static data member template.

The example given is:

struct matrix_constants {  
 template <typename T>   
  using pauli = hermitian_matrix<T, 2>;

Yet all of the following definitions give an error:

struct foo
{
    template <typename T>
    T pi = T{3.14};
};

template <typename T>
struct foo2
{
    template <typename U = T>
    U pi = U{3.14};
};

template <typename T>
struct foo3
{
    template <T>
    T pi = 42;
};

error: member 'pi' declared as a template

What gives?

Answer 1

EDIT: The committee has spoken, Clang is correct to require the static keyword for static data member templates. The examples given in 14/1 are not correct. Hopefully, the next revision of the working draft will remove the ambiguity from the text.

---

This seems to be a bug in Clang, but the wording in the draft standard is ambiguous. I believe the intention is that the keyword static is implicit. If that was not the intention, presumably the standard wording would be more along the lines of "A variable template at class scope must be a static data member template." instead of "A variable template at class scope is a static data member template." (N3797 §14/1) The (admittedly non-normative) example given in §14/1 declares three class member variable templates, none with the static keyword:

struct matrix_constants {
  template<class T>
   using pauli = hermitian_matrix<T, 2>;
  template<class T>
   constexpr pauli<T> sigma1 = { { 0, 1 }, { 1, 0 } };
  template<class T>
   constexpr pauli<T> sigma2 = { { 0, -1i }, { 1i, 0 } };
  template<class T>
   constexpr pauli<T> sigma3 = { { 1, 0 }, { -1, 0 } };
};

The example in 14.5.1.3 Static data members of class templates [temp.static]/1 notably does use static:

struct limits {
  template<class T>
    static const T min; // declaration
};

template<class T>
  const T limits::min = { }; // definition

so at the very least it's not forbidden to do so.

As @RichardSmith states in his comment, the actual normative text of the section contradicts the example. They write Clang to the text of the standard, so the example is diagnosed as ill-formed. The committee is aware that the wording for variable templates needs some help in various places, so I'm sure there will be some cleanup in the next draft / C++14.