c++ - Does casting to a pointer to a template instantiate that template?

Question

static_cast<the_template<int>*>(0) - does this instantiate the_template with type int?

The reason for asking is the following code, which will error at linking time with an undefined reference to check_error<char>(void*, long) with Clang and GCC 4.4.5, indicating that it does not instantiate the template. MSVC and GCC 4.5.1 however compile and link just fine, leading to the believe that it does instantiate the template. However, if you leave out the cast, MSVC and GCC (both 4.4.5 and 4.5.1) will error on check_error<char> only (the wanted behaviour), while Clang will error on both calls. Normally I believe Clang when it comes to conforming stuff, but I wonder:

Which compiler is correct and what does the standard say about it?

---

#include <type_traits>

template<class T>
void check_error(void*, long);

template<class T>
struct foo{
  template<class U>
  friend typename std::enable_if<
    std::is_same<T,U>::value
  >::type check_error(foo<T>*, int){}
};

template struct foo<int>;

int main()
{
  check_error<int>(static_cast<foo<int>*>(0), 0);
  check_error<char>(static_cast<foo<char>*>(0), 0);
}

Answer 1

It is not the cast that instantiates the class template specialization, but the function call, because the argument triggers ADL . The instantiation is done because the completeness of it may affect the semantics of the program.

That clang does not follow the spec here is known and a PR was sent by me some time ago. See http://llvm.org/bugs/show_bug.cgi?id=9440