Multiple expansions of multiple parameter packs in the same expression

Question

I want to ask if the following code is valid.

I wonder about the possibility to expand parameter packs multiple times in one expression.

#include <iostream>
#include <tuple>

class ExpandWithConstructor
{
    public:
        template <typename ... T>
        ExpandWithConstructor( T... args) { }
};

template <typename T>
int PrintArgs( T arg )
{
    std::cout << arg  << ", ";
    return 0;
}

template <typename Head, typename ... T>
class DebugPrinter: public DebugPrinter<T...>
{
    public:
        DebugPrinter() { }

        template< typename ...Y>
        DebugPrinter( Y ... rest ) 
        {   
            std::cout << "Construction of: " << __PRETTY_FUNCTION__ << " Values: " ;
            ExpandWithConstructor{PrintArgs( rest)...};
            std::cout << std::endl;
        }   

};

template <typename Head>
class DebugPrinter< Head >
{   
    public:
};  

template <typename ... T>
class TypeContainer: public std::tuple<T...>
{
    public:
        TypeContainer(T... args):std::tuple<T...>(args...){};
};

template <typename... T1> class CheckVariadic;

template <typename... T1, typename ...T2>
class CheckVariadic< TypeContainer<T1...>, TypeContainer<T2...>> :
          public DebugPrinter< T1, T2, T1...>...
{
    public:
        CheckVariadic( T1... args1, T2... args2, T1... args3): DebugPrinter< T1, T2, T1...>(args1, args2..., args1)... {}
};


int main()
{
    CheckVariadic< TypeContainer<int,float>, TypeContainer<char, void*>> checkVariadic1{ 1,2.2,'c',(void*)0xddddd,5,6.6,};
}

As you can see the code uses : DebugPrinter< T1, T2, T1...>...

if T1 is given with "int,float" and T2 is "char,void*" which expands to

DebugPrinter< T1, T2, int, float>...

which expands to

DebugPrinter< int, char, int, float>
DebugPrinter< float, void*, int, float>

The same expansion goes with:

 DebugPrinter< T1, T2, T1...>(args1, args2..., args1)...

The code compiles with clang3.3 but NOT with gcc4.8.1 so I want to ask if the code is valid or not.

Update: gcc 7.2 still did not compile the code.

Answer 1

Yes your code is perfectly valid. A pack expansion consists of a pattern and an ellipsis and can appear in a pattern of another pack expansion. In paragraph §14.5.3/5 of the standard you will find:

[...] An appearance of the name of a parameter pack is only expanded by the innermost enclosing pack expansion. The pattern of a pack expansion shall name one or more parameter packs that are not expanded by a nested pack expansion; [...]

A pack expansion can be used in any context mentioned in §14.5.3/4. Given your example:

DebugPrinter< T1, T2, T1...>...

Both pack expansions are valid. The context of the first one is a template-argument-list while the second appears in a base-specifier-list.

The example provided by the standard text:

template<class ... Args>
void g(Args ... args) {                   // OK: Args is expanded by the function
                                          // parameter pack args
    f(const_cast<const Args*>(&args)...); // OK: “Args” and “args” are expanded
    f(5 ...);                             // error: pattern does not contain any
                                          // parameter packs
    f(args);                              // error: parameter pack “args” is not
                                          // expanded
    f(h(args ...) + args ...);            // OK: first “args” expanded within h,
                                          // second “args” expanded within f
}