Added ! operator for all monad types (related to #261).

Author: sakno

src/DotNext.Tests/OptionalTests.cs

@@ -359,4 +359,27 @@ public static async Task FlattenTask()
         await ThrowsAsync<InvalidOperationException>(static () => Task.FromResult(Optional.None<int>()).Flatten());
         Equal(42, await Task.FromResult(Optional.Some(42)).Flatten());
     }
+
+    [Fact]
+    public static void MiscOperators()
+    {
+        Optional<int> result = 20;
+        False(!result);
+
+        if (result)
+        {
+        }
+        else
+        {
+            Fail("Optional has no value");
+        }
+        
+        result = Optional<int>.None;
+        True(!result);
+
+        if (result)
+        {
+            Fail("Optional has value");
+        }
+    }
 }

src/DotNext/IOptionMonad.cs

@@ -84,8 +84,14 @@ public static virtual bool operator true(in TSelf container)
     /// </summary>
     /// <param name="container">The container to check.</param>
     /// <returns><see langword="true"/> if this container has no value; otherwise, <see langword="false"/>.</returns>
-    public static virtual bool operator false(in TSelf container)
-        => container.HasValue is false;
+    public static virtual bool operator false(in TSelf container) => !container;
+
+    /// <summary>
+    /// Checks whether the container has no value.
+    /// </summary>
+    /// <param name="container">The container to check.</param>
+    /// <returns><see langword="true"/> if this container has no value; otherwise, <see langword="false"/>.</returns>
+    public static virtual bool operator !(in TSelf container) => container.HasValue is false;
 
     /// <summary>
     /// Returns the value if present; otherwise return default value.

src/DotNext/Optional.cs

@@ -772,16 +772,22 @@ public bool Equals(object? other, IEqualityComparer comparer)
     /// </summary>
     /// <param name="optional">The container to check.</param>
     /// <returns><see langword="true"/> if this container has value; otherwise, <see langword="false"/>.</returns>
-    /// <see cref="HasValue"/>
-    [MemberNotNullWhen(true, nameof(ValueOrDefault))]
+    /// <seealso cref="HasValue"/>
     public static bool operator true(in Optional<T> optional) => optional.HasValue;
 
     /// <summary>
     /// Checks whether the container has no value.
     /// </summary>
     /// <param name="optional">The container to check.</param>
     /// <returns><see langword="true"/> if this container has no value; otherwise, <see langword="false"/>.</returns>
-    /// <see cref="HasValue"/>
-    [MemberNotNullWhen(false, nameof(ValueOrDefault))]
-    public static bool operator false(in Optional<T> optional) => optional.kind < NotEmptyValue;
+    /// <seealso cref="HasValue"/>
+    public static bool operator false(in Optional<T> optional) => !optional;
+
+    /// <summary>
+    /// Checks whether the container has no value.
+    /// </summary>
+    /// <param name="optional">The container to check.</param>
+    /// <returns><see langword="true"/> if this container has no value; otherwise, <see langword="false"/>.</returns>
+    /// <seealso cref="HasValue"/>
+    public static bool operator !(in Optional<T> optional) => optional.kind < NotEmptyValue;
 }