Templated initialization_list - why does lookup fail

Question

Given

#include <utility>
template <typename T1, typename T2, typename T3>
void foo(std::initializer_list<std::pair<T1, T2>> _a, std::initializer_list<T3> _b) {
    /* ... */
}

int main() {
        foo({{1,2},{3,4}},{1,2,3,4,5});
}

why does gcc 4.6 produce the error

error: no matching function for call to ‘foo(<brace-enclosed initializer list>, <brace-enclosed initializer list>)'

note: candidate is

note: template<class T1, class T2, class T3> void foo(std::initializer_list<std::pair<T1, T2> >, std::initializer_list<T3>)

Update Now works when

template <typename T1, typename T2>
std::pair<T1, T2> p(const T1& _1, const T2& _2) {
    return std::make_pair(_1, _2);
}

is added and {{1,2},{3,4}} is replaced with {p(1,2), p(3,4)}.

Answer 1

What you're trying to do is basically chained conversion/invocation, which is not allowed in C++.

First you want to convert this,

{{1,2},{3,4}}

into this (while deducing the type argument),

{std::pair<int,int>{1,2},std::pair<int,int>{3,4}}

which then into std::initializer_list. It is not allowed.

You've to do one conversion yourself:

foo( {std::pair<int,int>{1,2},std::pair<int,int>{3,4}}, {1,2,3,4,5});

then it will compile : http://ideone.com/bRJqV