#include <array>
 
// Assume this comes from some included file
constexpr size_t elementCount = 5;
 
template<class first, class ... T>
std::array<first, 1 + sizeof...(T)> makeArray(first e1, T ... rest) {
	static_assert(1 + sizeof...(T) == elementCount, "Must match elementCount");
    return {  std::forward<first>(e1), std::forward<T>(rest) ...};
}
 
int main() {
    // This works.
    const auto lookup = makeArray(2, 4, 6, 8, 10);
 
    // This doesn't. :-)
    //const auto lookup2 = makeArray(2, 4, 6);
}

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