No "redefinition of default parameter error" for class template member function?

Why does the following give no compilation error?:

// T.h

template<class T> class X
{
public:
    void foo(int a = 42);
};

// Main.cpp

#include "T.h"
#include <iostream>

template<class T> void X<T>::foo(int a = 13)
{
    std::cout << a << std::endl;
}

int main()
{
    X<int> x;
    x.foo();   // prints 42
}

It seems as开发者_如何学编程 though the 13 is just silently ignored by the compiler. Why is this?

The cooky thing is that if the class template definition is in Main.cpp instead of a header file, I do indeed get the default parameter redefinition error.

Now I know the compiler will complain about this if it were just an ordinary (non-template) function.

What does the standard have to say about default parameters in class template member functions or function templates?

8.3.6 §6 The default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition.
[Example:

class C {
    void f(int i = 3);
    void g(int i, int j = 99);
};
void C::f(int i = 3) // error: default argument already
{ }                  // specified in class scope
void C::g(int i = 88, int j) // in this translation unit,
{ }                          // C::g can be called with no argument

--end example]

According to the standard, it should give you an error.