The following example does not work:
#include <iostream>
template <int index, template <typename> class Head,
template <typename> class... Tail>
struct TemplateTupleElement {
template <typename T>
using Type =
typename TemplateTupleElement<index - 1, Tail...>::template Type<T>;
};
template <template <typename> class Head, template <typename> class... Tail>
struct TemplateTupleElement<0, Head, Tail...> {
template <typename T>
using Type = Head<T>;
};
template <typename T>
class Dummy {
};
template <template <typename> class T>
class TemplateDummy {
public:
static void print() {
std::cout << "Template" << std::endl;
}
};
template <>
class TemplateDummy<Dummy> {
public:
static void print() {
std::cout << "Specialization" << std::endl;
}
};
int main(int argc, char* argv[]) {
TemplateDummy<TemplateTupleElement<0, Dummy, Dummy>::Type>::print();
TemplateDummy<TemplateTupleElement<1, Dummy, Dummy>::Type>::print();
return 0;
}
I expected the result would be:
Specialization
Specialization
However, the output of a program compiled with g ++ ((Ubuntu 5.3.0-3ubuntu1 ~ 14.04) 5.3.0 20151204):
Specialization
Template
And exit from Clang (version 3.4-1ubuntu3):
Template
Template
I understand that internally, the compiler presents two template expressions differently:
DummyTemplateTupleElement<1, Dummy, Dummy>::Type
Why is the second expression not “simplified” in the first g ++ expression, and why is Klang not displaying the expression correctly using tuple indices?
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