This is a weird bug ```cpp #include <array> #include <tuple> template<class Tuple> auto foo() { constexpr auto N = std::tuple_size_v<Tuple>; constexpr auto M = []<std::size_t... Is> (std::index_sequence<Is...>) { std::array<std::size_t, N> a{Is...}; // ok std::array<std::size_t, N> b{}; // not ok return 0; }(std::make_index_sequence<N>{}); }; ``` https://godbolt.org/z/h7f1sTW6j