Split classes into neater header units

include/spinscale/co/promiseReturnOps.h

@@ -0,0 +1,38 @@
+#ifndef PROMISE_RETURN_OPS_H
+#define PROMISE_RETURN_OPS_H
+
+#include <type_traits>
+#include <utility>
+
+#include <spinscale/co/returnValues.h>
+
+namespace sscl::co {
+
+/**	`return_value` / `return_void` only. ThreadTag is not a template parameter here:
+ *	for tagged promises, PromiseType is `TaggedPostingPromise<T, ThreadTag>`.
+ */
+template <typename PromiseType, typename T, bool IsVoid = std::is_void_v<T>>
+struct PromiseReturnOps;
+
+template <typename PromiseType, typename T>
+struct PromiseReturnOps<PromiseType, T, false>
+{
+	void return_value(T returnValue) noexcept
+	{
+		static_cast<PromiseType *>(this)->returnValues.myReturnValue =
+			std::move(returnValue);
+	}
+};
+
+template <typename PromiseType, typename T>
+struct PromiseReturnOps<PromiseType, T, true>
+{
+	void return_void() noexcept
+	{
+		return;
+	}
+};
+
+} // namespace sscl::co
+
+#endif // PROMISE_RETURN_OPS_H