Static member access in constant expressions

Accessing static class member functions or variables, can be done in two ways: through an object (obj.member_fun() or obj.member_var) or through the class (Class::member_fun() or Class::member_var). However, in constexpr functions, Clang gives an error on the object access and requires to use class access:

struct S 
{
    constexpr static auto s_v = 42;    
    constexpr static auto v() { return s_v; }
};

#define TEST 1

constexpr auto foo(S const& s [[maybe_unused]]) 
{
#if TEST
    constexpr auto v = s.v();   // ERROR for clang, OK for gcc
#else    
    constexpr auto v = S::v();  // OK for clang and gcc
#endif
    return v;
}

constexpr auto bar(S const& s [[maybe_unused]])
{
#if TEST   
    constexpr auto v = s.s_v;   // ERROR for clang, OK for gcc
#else    
    constexpr auto v = S::s_v;  // OK for clang and gcc
#endif    
    return v;
}

int main() {}

Live Example compiled with -std=c++1z and #define TEST 1 for Clang 5.0 SVN, with error message:

Start
prog.cc:12:24: error: constexpr variable 'v' must be initialized by a constant expression
    constexpr auto v = s.v();   // ERROR for clang, OK for gcc
                       ^~~~~
prog.cc:22:24: error: constexpr variable 'v' must be initialized by a constant expression
    constexpr auto v = s.s_v;   // ERROR for clang, OK for gcc
                       ^~~~~
2 errors generated.
1
Finish

Question: is this is a Clang bug, or is gcc too liberal in accepting both syntax forms for static member access in a constexpr function?