SFINAE difference with parameter versus template arg [duplicate]

When compiled with g++ using C++-11, the following code snippet does not compile:

class A
{
public:
    template <typename T, typename = typename std::enable_if<T::value>::type>
    A(T const &);

    template <typename T, typename = typename std::enable_if<!T::value>::type>
    A(T const &);
};

Specifically, I get a redefinition error on the second constructor. However, this snippet compiles fine:

class A
{
public:
    template <typename T>
    A(T const &, typename std::enable_if<T::value>::type * = nullptr);

    template <typename T>
    A(T const &, typename std::enable_if<!T::value>::type * = nullptr);
};

All I've done is move the enable_if from the template list to the parameter list. Given this:

(1) What specific rule in the standard allows the second but not the first?

(2) What is the underlying rationale for the two snippets to be treated differently?