Having trouble passing multiple initializer lists to variadic function template

Question

I don\'t understand the error message when trying to pass a variable number of initializer lists:

template
void foo(Values...)
{
}
         


        

Answer 1

This is bad. Consider a simple print() utility:

  template 
  void print ( Args&&... args) ;

All of this would work:

print("Word", 12, 13.0f, true );

Tuple also works (ignore the implementation required):

auto tup = std::make_tuple("A", true, 42f ) ;    
print("\nTuple I can pass it in ", tup );

But none of this works

print({1,2,3}); // spurious error messages
print({1}, {2}, {3}); // also 
print("\nThe tuple: ", {12, 34, 56 } ) ; //also

The above "solution" does not help too:

template
inline  void print(const std::initializer_list&... il_);

This (as above) does not give usable print() utility :

print("\nMy list is:\t", {1,2,3,4}) ; // error: function print() does not take 2 arguments?

Is it something obvious that is missing here? I would like to mix anything in the call to print() as it's declaration implies.

Anybody?

[Edit 2017-11-08]

Somebody has suggested

 print("\nMy list is:\t", std::initializer_list{1,2,3,4}) ;

And to somewhat remedy the pain of this, I am crushed to admit I have defined this macro "helper"

#define DBJ_IL(T,...) (std::initializer_list{__VA_ARGS__})

Usage:

print("\nMy list is:\t", DBJ_IL(int,1,2,3,4)) ;

But alas, MSVC 14.11.25503 (the latest as of time of this writing) can not compile this. With errors coming from

    1>c:\program files (x86)\microsoft visual 
    studio\2017\community\vc\tools\msvc\14.11.25503\include\utility(415): 
    error C2027: use of undefined type 'std::tuple_size<_Ty>'
   1>        with
   1>        [
   1>            _Ty=std::initializer_list
   1>        ]
   1>c:\program files (x86)\microsoft visual 
      studio\2017\community\vc\tools\msvc\14.11.25503\include\utility(415): 
      note: see declaration of 'std::tuple_size<_Ty>'
   1>        with
   1>        [
   1>            _Ty=std::initializer_list
   1>        ]
   1>c:\program files (x86)\microsoft visual 
    studio\2017\community\vc\tools\msvc\14.11.25503\include\tuple(1051): 
     note: see reference to variable template 'const ::size_t 
     tuple_size_v >' being compiled

I am sure nobody want's the rest of the MSVC error dump ... Is it me or is it them?

Doing the print() as generic lambda does not solve anything of course.

/*
forget templates
*/
namespace dbj { namespace {
  auto print = [](auto... param)
  {
   if constexpr (sizeof...(param) > 0) {
    char dummy[sizeof...(param)] = { 
          (( std::cout << param), 0)... 
        };
     }
  };
} }

Even if one passes single and simple init list this wont compile with the same error as above ...

 dbj::print({1,2,3}) ; // msvc compilation error

I know C++17 type deduction of init lists is strengthened and improved, but I can not see in there anything to help me understand is this doable at all?

At last it seems it should be.