c++ - Derived class in multiple inheritance behaves like an aggregate -


i following code (also found in cppreference http://en.cppreference.com/w/cpp/utility/variant/visit) compile (https://wandbox.org/permlink/stcfki0vqlf49bxr)

#include <type_traits> #include <utility>  template <typename... types> struct overload : public types... {     using types::operator()...; }; template <typename... types> auto make_overload(types&&... instances) {     return overload<std::decay_t<types>...>{std::forward<types>(instances)...}; }  int main() {     auto overloaded = make_overload([](int) {}, [](double) {});     static_cast<void>(overloaded); }

how code above compile in c++17? not compile in c++14. happening behind scenes? why variadic using declaration not work in c++14? new feature this?

as can read in parameter_pack,

introduced in c++17

the following list of allowed contexts: [...]

using-declarations
in using declaration, ellipsis may appear in list of declarators, useful when deriving parameter pack:

template <typename... bases> struct x : bases... {     using bases::g...; }; x<b, d> x; // ok: b::g , d::g introduced

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