Multiple function definition for templated function due to inheritance

Question

I cannot compile my program with VS2015 due to a function using a templated parameter and inheritance.

The error is this one.

I'm trying to achieve the following :

class A
{
    //do something
};

class B : public A
{
    //do something
};

template <typename T>
class Foo {
    template <typename T>
    friend void function(Foo<T> & sm) {
        //do something
    }
};

void main()
{
    Foo<A> test;
    Foo<B> test2;
};

I do understand the meaning of the error, but I do not understand why it actually happens.

I suppose function is created with two different signatures :

void function(Foo<A> & sm); and void function(Foo<B> & sm);

How is that a multi-definition?

EDIT - The full error message : Error C2995 'void function(Foo<T> &)': function template has already been defined

EDIT² - From scratch

Answer 1

Both Clang and MS have the same complaint. Remove the second template specifier and it will compile.

class A{};
class B : public A{};

template <typename T>
class Foo {

//  template <typename T>
    friend void function(Foo<T> & sm) {
    }
};

int main()
{
    Foo<A> test;
    Foo<B> test2;
};

T is already specified for the class Foo so its friend function is covered. You would us a second template if there were a difference at the function, like:

class A{};
class B : public A{};

template <typename T>
class Foo {

    template <typename U>
    friend void function(Foo<T> & sm, U another) {
    }
};

int main()
{
    Foo<A> test;
    Foo<B> test2;
};