beman.task migration into execution

include/beman/execution/detail/sub_visit.hpp

@@ -0,0 +1,44 @@
+// include/beman/execution/detail/sub_visit.hpp                            -*-C++-*-
+// ----------------------------------------------------------------------------
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// ----------------------------------------------------------------------------
+
+#ifndef INCLUDED_BEMAN_EXECUTION_DETAIL_SUB_VISIT
+#define INCLUDED_BEMAN_EXECUTION_DETAIL_SUB_VISIT
+
+#include <utility>
+#include <variant>
+#include <cstddef>
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution::detail {
+/*
+ * \brief Helper function creatig thunks for a variant visit.
+ * \headerfile beman/execution/task.hpp <beman/execution/task.hpp>
+ * \internal
+ */
+template <std::size_t Start, typename Fun, typename Var, std::size_t... I>
+void sub_visit_thunks(Fun& fun, Var& var, std::index_sequence<I...>) {
+    using thunk_t = void (*)(Fun&, Var&);
+    static constexpr thunk_t thunks[]{(+[](Fun& f, Var& v) { f(std::get<Start + I>(v)); })...};
+    thunks[var.index() - Start](fun, var);
+}
+
+/*
+ * \brief Helper function visiting a suffix of variant options
+ * \headerfile beman/execution/task.hpp <beman/execution/task.hpp>
+ * \internal
+ */
+template <std::size_t Start, typename... T>
+void sub_visit(auto&& fun, std::variant<T...>& v) {
+    if (v.index() < Start)
+        return;
+    sub_visit_thunks<Start>(fun, v, std::make_index_sequence<sizeof...(T) - Start>{});
+}
+
+} // namespace beman::execution::detail
+
+// ----------------------------------------------------------------------------
+
+#endif