Correct the GenericToast props to show the two modes of operation

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/@types/common.ts

@@ -0,0 +1,19 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Based on https://stackoverflow.com/a/53229857/3532235
+export type Without<T, U> = {[P in Exclude<keyof T, keyof U>] ? : never}
+export type XOR<T, U> = (T | U) extends object ? (Without<T, U> & U) | (Without<U, T> & T) : T | U;

src/components/views/toasts/GenericToast.tsx

@@ -17,17 +17,21 @@ limitations under the License.
 import React, {ReactChild} from "react";
 
 import FormButton from "../elements/FormButton";
+import {XOR} from "../../../@types/common";
 
 interface IProps {
     description: ReactChild;
     acceptLabel: string;
-    rejectLabel?: string;
 
     onAccept();
-    onReject?();
 }
 
-const GenericToast: React.FC<IProps> = ({description, acceptLabel, rejectLabel, onAccept, onReject}) => {
+interface IPropsExtended extends IProps {
+    rejectLabel: string;
+    onReject();
+}
+
+const GenericToast: React.FC<XOR<IPropsExtended, IProps>> = ({description, acceptLabel, rejectLabel, onAccept, onReject}) => {
     return <div>
         <div className="mx_Toast_description">
             { description }