ADD a warning for #5781

lib/client-api/src/client_api.js

@@ -134,6 +134,7 @@ export default class ClientApi {
 
     const localDecorators = [];
     let localParameters = {};
+    let hasAdded = false;
     const api = {
       kind,
     };
@@ -148,6 +149,7 @@ export default class ClientApi {
     });
 
     api.add = (storyName, storyFn, parameters) => {
+      hasAdded = true;
       const { _globalParameters, _globalDecorators } = this;
       const id = toId(kind, storyName);
 
@@ -215,6 +217,12 @@ export default class ClientApi {
     };
 
     api.addDecorator = decorator => {
+      if (hasAdded) {
+        console.warn(`You have added a decorator to the kind '${kind}' after a story has already been added.
+In Storybook 4 this applied the decorator only to subsequent stories. In Storybook 5+ it applies to all stories.
+This is probably not what you intended. Read more here: https://github.com/storybooks/storybook/blob/master/MIGRATION.md`);
+      }
+
       localDecorators.push(decorator);
       return api;
     };