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<typename... Values>
void foo(Values...)
{
}

int main()
{
    foo(1, 2, 3, "hello", 'a');   // OK

    foo({1}, {2, 3});             // ERROR
}

The error message complains about too many arguments:

prog.cpp: In function ‘int main()’:
prog.cpp:9:20: error: too many arguments to function
                      ‘void foo(Values ...) [with Values = {}]’
     foo({1}, {2, 3});
                    ^
prog.cpp:2:6: note: declared here
 void foo(Values...)
      ^

However, should I not be able to pass as many arguments as I want? [ideone link]

Answer 1

The problem is likely deducibility. {} could be uniform initializers to any of the arguments.

This works:

#include <initializer_list>

template<typename... Values>
void foo(std::initializer_list<Values>... args)
{
}

template<typename... Values>
void foo(Values&&... args)
{
}

int main()
{    
    foo(1, 2, 3, "hello", 'a');
    foo({1}, {2, 3});
}

See it Live on Coliru

Answer 2

The issue is not with the varadic arguments, but that the compiler cannot deduce the type of a brace enclosed initializer list, except for the case where you've declare the parameter of std::initializer_list<T>

§ 14.8.2.1 Template argument deduction is done by comparing each function template parameter type (call it P) with the type of the corresponding argument of the call (call it A) as described below. If removing references and cv-qualifiers from P gives std::initializer_list for some P0 and the argument is an initializer list (8.5.4), then deduction is performed instead for each element of the initializer list, taking P0 as a function template parameter type and the initializer element as its argument. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (14.8.2.5).

There's even an example right below

template<class T> void g(T);
g({1,2,3}); // error: no argument deduced for T