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+An unary operator was used on a type which doesn't implement it.
+
+Erroneous code example:
+
+```compile_fail,E0600
+enum Question {
+    Yes,
+    No,
+}
+
+!Question::Yes; // error: cannot apply unary operator `!` to type `Question`
+```
+
+In this case, `Question` would need to implement the `std::ops::Not` trait in
+order to be able to use `!` on it. Let's implement it:
+
+```
+use std::ops::Not;
+
+enum Question {
+    Yes,
+    No,
+}
+
+// We implement the `Not` trait on the enum.
+impl Not for Question {
+    type Output = bool;
+
+    fn not(self) -> bool {
+        match self {
+            Question::Yes => false, // If the `Answer` is `Yes`, then it
+                                    // returns false.
+            Question::No => true, // And here we do the opposite.
+        }
+    }
+}
+
+assert_eq!(!Question::Yes, false);
+assert_eq!(!Question::No, true);
+```