If a template declaration references a member of a dependent type, it is only possible to alert the user to the non-existence of this member during type instantiation for a given set of template arguments. For example:

template<class T>
class foo
{
foo(int *a = T::A);     // we expect that "A" is a data member
};
struct bar { };
foo<bar> y;// error: "bar" has no member named "A"