works with void-returning awaitables

examples/Makefile

@@ -5,7 +5,7 @@ CXXFLAGS=-I$(IDIR) -std=c++20 -stdlib=libc++
 
 LIBS=-pthread
 
-all: hello_world
+all: hello_world hello_coro
 
 execution: $(IDIR)/execution.hpp
 

examples/task.hpp

@@ -36,7 +36,13 @@ struct task {
     void await_resume() const noexcept {
     }
   };
-  struct promise_type {
+  struct _promise_base {
+    void return_value(T value) noexcept {
+      data_.template emplace<1>(std::move(value));
+    }
+    std::variant<std::monostate, T, std::exception_ptr> data_{};
+  };
+  struct promise_type : _promise_base {
     task get_return_object() noexcept {
       return task(coro::coroutine_handle<promise_type>::from_promise(*this));
     }
@@ -47,12 +53,8 @@ struct task {
       return {};
     }
     void unhandled_exception() noexcept {
-      data_.template emplace<2>(std::current_exception());
-    }
-    void return_value(T value) noexcept {
-      data_.template emplace<1>(std::move(value));
+      this->data_.template emplace<2>(std::current_exception());
     }
-    std::variant<std::monostate, T, std::exception_ptr> data_{};
     coro::coroutine_handle<> parent_{};
   };
 
@@ -77,7 +79,8 @@ struct task {
     T await_resume() const {
       if (t.coro_.promise().data_.index() == 2)
         std::rethrow_exception(std::get<2>(t.coro_.promise().data_));
-      return std::get<T>(t.coro_.promise().data_);
+      if constexpr (!std::is_void_v<T>)
+        return std::get<T>(t.coro_.promise().data_);
     }
   };
 
@@ -91,3 +94,12 @@ struct task {
   {}
   coro::coroutine_handle<promise_type> coro_;
 };
+
+template<>
+struct task<void>::_promise_base {
+  struct _void {};
+  void return_void() noexcept {
+    data_.template emplace<1>(_void{});
+  }
+  std::variant<std::monostate, _void, std::exception_ptr> data_{};
+};

include/execution.hpp

@@ -282,9 +282,9 @@ namespace std::execution {
           // The 'co_yield' expression then invokes this lambda
           // after the coroutine is suspended so that it is safe
           // for the receiver to destroy the coroutine.
-          auto fn = [&](auto&&... result) {
+          auto fn = [&](auto&&... r) {
             return [&] {
-              set_value((R&&) r, (__await_result_t<A>&&) result...);
+              set_value((R&&) r, (add_rvalue_reference_t<__await_result_t<A>>) r...);
             };
           };
           if constexpr (is_void_v<__await_result_t<A>>)