extern template 'inconsistent explicit instantiations'

Question

Given

#include <vector>

// Case I: error

error C2961: 'std::vector>': inconsistent explicit instantiations, a previous explicit instantiation did not specify 'extern template'

template class std::vector<int>;
extern template class std::vector<int>;


// Case II: fine
//extern template class std::vector<int>;
//template class std::vector<int>;

// Case III: fine
//extern template class std::vector<int>;
//template class std::vector<int>;
//template class std::vector<int>;

// Case IV: fine
//extern template class std::vector<int>;
//extern template class std::vector<int>;
//template class std::vector<int>;
//template class std::vector<int>;


int main()
{

}

I build the code with VS2015 Version 4.6 and see the C2961 error. However, I don't see the error with compiler http://gcc.godbolt.org/.

Is this a bug with VS2015?

Answer 1

MSVC and Clang are right to issue an error in this case. [temp.explicit]/11 in the standard says:

If an entity is the subject of both an explicit instantiation declaration and an explicit instantiation definition in the same translation unit, the definition shall follow the declaration. [...]

template class std::vector<int>; is an explicit instantiation definition. The one with the extern is an explicit instantiation declaration.

---

Note that explicitly instantiating std::vector<int> is ill-formed in any case according to [namespace.std]/2:

[...] A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

Also note that cases III and IV are also ill-formed according to [temp.spec]/5:

For a given template and a given set of template-arguments,

• an explicit instantiation definition shall appear at most once in a program,

[...]

An implementation is not required to diagnose a violation of this rule.