import from UniRx and some modified.

Assets/UniRx.Async/UniTaskVoid.cs

@@ -0,0 +1,48 @@
+#if CSHARP_7_OR_LATER || (UNITY_2018_3_OR_NEWER && (NET_STANDARD_2_0 || NET_4_6))
+#pragma warning disable CS1591
+
+using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using UniRx.Async.CompilerServices;
+
+namespace UniRx.Async
+{
+    [AsyncMethodBuilder(typeof(AsyncUniTaskVoidMethodBuilder))]
+    public struct UniTaskVoid
+    {
+        public void Forget()
+        {
+        }
+
+        [DebuggerHidden]
+        public Awaiter GetAwaiter()
+        {
+            return new Awaiter();
+        }
+
+        public struct Awaiter : ICriticalNotifyCompletion
+        {
+            [DebuggerHidden]
+            public bool IsCompleted => true;
+
+            [DebuggerHidden]
+            public void GetResult()
+            {
+                UnityEngine.Debug.LogWarning("UniTaskVoid can't await, always fire-and-forget. use Forget instead of await.");
+            }
+
+            [DebuggerHidden]
+            public void OnCompleted(Action continuation)
+            {
+            }
+
+            [DebuggerHidden]
+            public void UnsafeOnCompleted(Action continuation)
+            {
+            }
+        }
+    }
+}
+
+#endif