Implicit use of initializer_list

§[dcl.init.list] 8.5.4/2:

The template std::initializer_list is not predefined; if the header <initializer_list> is not included prior to a use of std::initializer_list — even an implicit use in which the type is not named (7.1.6.4) — the program is ill-formed.

Does that mean this program is ill-formed?

#include <vector>
int main() {
    // uses vector::vector(initializer_list<T>, const Allocator&) constructor
    std::vector<int> v = {1, 2, 3};
}

Solution

Your program is not ill-formed because <vector> is guaranteed to include <initializer_list> (the same is true for all standard library containers)

§23.3.1 [sequences.general]

Header <vector> synopsis

#include <initializer_list>
...

Searching the standard for #include <initializer_list> reveals the header is included along with the following headers

<utility>
<string>
<array>
<deque>
<forward_list>
<list>
<vector>
<map>
<set>
<unordered_map>
<unordered_set>
<queue>
<stack>
<algorithm>
<random>
<valarray>
<regex>