Go home clang, you're drunk.

gistfile1.txt

 % cat template.cpp
template<class T>
class X {
  T f(T x) { return x; }

  T foo(void);
};

template<class T>
T X::foo(void)
{
  return f(5);
}

% ./clang template.cpp -o template -Wall
template.cpp:9:3: error: 'X' is not a class, namespace, or enumeration
T X::foo(void)
  ^
template.cpp:2:7: note: 'X' declared here
class X {
      ^
1 error generated.