::: spoiler alt ```c++ #include // from https://stackoverflow.com/a/8625010/12469275 // cmath's sqrt is constexpr from c++26 constexpr std::size_t isqrt_impl (std::size_t sq, std::size_t dlt, std::size_t value){ return sq <= value ? isqrt_impl(sq+dlt, dlt+2, value) : (dlt >> 1) - 1; } constexpr std::size_t isqrt(std::size_t value){ return isqrt_impl(1, 3, value); } // because pack indexing is only in c++26 template struct At; template struct At {}; template struct At<0, void, V0, V...> { static const std::size_t value = V0; }; template struct At, V0, V...> { static const std::size_t value = At::value; }; template struct Add { static const std::size_t value = A + B; }; template typename R, std::size_t I, typename _, std::size_t... V> struct _ReduceFor; template typename R, std::size_t I, std::size_t... V> struct _ReduceFor, V...> { typedef R::value,_ReduceFor::type::value> type; }; template typename R, std::size_t I, std::size_t... V> struct _ReduceFor= end), void>, V...> { typedef std::integral_constant type; }; template typename R, std::size_t I, std::size_t... V> using ReduceFor = _ReduceFor; template typename V, typename T, typename _> struct AllFor; template typename V, typename T> struct AllFor> { typedef std::enable_if_t::type, bool>::value, typename AllFor::type> type; }; template typename V, typename T> struct AllFor= end), void>> { typedef T type; }; template typename V, typename T> using AllFor_t = typename AllFor::type; template struct ValidRows { template struct Inner { typedef std::enable_if_t::type::value == S * (S*S + 1)/2, T2> type; }; typedef AllFor_t<0, S*S, S, Inner, T> type; }; template struct ValidColumns { template struct Inner { typedef std::enable_if_t::type::value == S * (S*S + 1)/2, T2> type; }; typedef AllFor_t<0, S, 1, Inner, T> type; }; template struct ValidDiags { typedef std::enable_if_t::type::value == S * (S*S + 1)/2 && ReduceFor::type::value == S * (S*S + 1)/2, T> type; }; template struct Unique; template struct Unique { template struct Inner { typedef std::enable_if_t::value,T2> type; }; typedef AllFor_t<0,sizeof...(V),1,Inner,T> type; }; template struct Unique { typedef T type; }; template struct InRange { template struct Inner { typedef typename std::enable_if<1 <= At::value && At::value <= sizeof...(V), T2>::type type; }; typedef AllFor_t<0,sizeof...(V),1,Inner,T> type; }; template struct Grid { static const std::size_t S = isqrt(sizeof...(V)); typedef std::enable_if_t::type, V...>::type, V...>::type, V...>::type, V...>::type> type; }; using ok = Grid::type; int main() {} ``` :::

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