add github workflow for test-pypi cross-OS wheel builds

Author: AlePalu

include/fdaPDE-cpp/fdaPDE/core/fdaPDE/src/utility/meta.h

@@ -23,14 +23,27 @@ namespace fdapde {
 namespace internals {
 
 // apply lambda F_ to each value in index pack {0, ..., N_ - 1}
-template <typename F_, int... Ns_>
-constexpr decltype(auto) apply_index_pack_impl(F_&& f, std::integer_sequence<int, Ns_...>) {
-    return f.template operator()<Ns_...>();
-}
-template <int N_, typename F_> constexpr decltype(auto) apply_index_pack(F_&& f) {
-    return internals::apply_index_pack_impl(std::forward<F_>(f), std::make_integer_sequence<int, N_>{});
-}
+template <int... Ns>
+struct index_tag {};
+
+    // 2. The implementation now takes the tag as an argument
+    template <typename F_, int... Ns_>
+    constexpr decltype(auto) apply_impl(F_&& f, index_tag<Ns_...>) {
+        // We pass the integers as types via the tag, not as template args to the call
+        return std::forward<F_>(f).template operator()<Ns_...>();
+    }
 
+    // 3. A helper to convert integer_sequence to our tag
+    template <typename F_, int... Ns_>
+    constexpr decltype(auto) sequence_to_tag(F_&& f, std::integer_sequence<int, Ns_...>) {
+        return apply_impl(std::forward<F_>(f), index_tag<Ns_...>{});
+    }
+
+template <int N_, typename F_>
+constexpr decltype(auto) apply_index_pack(F_&& f) {
+    return internals::sequence_to_tag(std::forward<F_>(f), std::make_integer_sequence<int, N_>{});
+}
+  
 template <int N_, typename F_> constexpr void for_each_index_in_pack(F_&& f) {
     [&]<int... Ns_>(std::integer_sequence<int, Ns_...>) {
         (f.template operator()<Ns_>(), ...);