Class template overloading across TUs

Question

Consider the following C++11 application:

A.cpp:

template<typename T>
struct Shape {
    T x;
    T area() const { return x*x; }
};

int testA() {
    return Shape<int>{2}.area();
}

B.cpp:

template<typename T, typename U = T>
struct Shape {
    T x;
    U y;
    U area() const { return x*y; }
};
int testB() {
    return Shape<int,short>{3,4}.area();
}

Main.cpp:

int testA();
int testB();
int main() {
   return testA() + testB();
}

Although it compiles (as long as A and B are in separate TUs), it doesn't look right, and I'm having trouble figuring out why.

Hence my Question: Does this violate ODR, overloading, or any other rule, and if so, what sections of the Standard are violated and why?

Answer 1

It is an ODR violation. Template names have linkage. And both those template names have external linkage, as [basic.link]/4 says:

An unnamed namespace or a namespace declared directly or indirectly within an unnamed namespace has internal linkage. All other namespaces have external linkage. A name having namespace scope that has not been given internal linkage above has the same linkage as the enclosing namespace if it is the name of

• [...]
• a template.

And on account of that, since both templates share a name, it means that [basic.def.odr]/5 applies:

There can be more than one definition of a [...] class template (Clause [temp]) [...] in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then

• each definition of D shall consist of the same sequence of tokens; and
• [...]

If D is a template and is defined in more than one translation unit, then the preceding requirements shall apply both to names from the template's enclosing scope used in the template definition ([temp.nondep]), and also to dependent names at the point of instantiation ([temp.dep]). If the definitions of D satisfy all these requirements, then the program shall behave as if there were a single definition of D. If the definitions of D do not satisfy these requirements, then the behavior is undefined.

Not the same sequence of tokens by a margin.

You can easily resolve it, as Jarod42 suggested, by putting both definitions of the templates into an unnamed namespace, thus giving them internal linkage.