core-plugin-api: allow unknown plugin outputs for backwards compatibility

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/core-app-api/src/app/createApp.tsx

@@ -35,6 +35,7 @@ import {
   BootErrorPageProps,
   ErrorBoundaryFallbackProps,
 } from './types';
+import { BackstagePlugin } from '@backstage/core-plugin-api';
 
 /**
  * The default config loader, which expects that config is available at compile-time
@@ -171,7 +172,7 @@ export function createApp(options?: AppOptions) {
   return new PrivateAppImpl({
     apis,
     icons,
-    plugins,
+    plugins: plugins as BackstagePlugin<any, any>[],
     components,
     themes,
     configLoader,

packages/core-app-api/src/app/types.ts

@@ -24,6 +24,7 @@ import {
   RouteRef,
   SubRouteRef,
   ExternalRouteRef,
+  PluginOutput,
 } from '@backstage/core-plugin-api';
 import { AppConfig } from '@backstage/config';
 import { AppIcons } from './icons';
@@ -131,6 +132,21 @@ export type AppRouteBinder = <
   >,
 ) => void;
 
+// Output from newer or older plugin API versions that might not be supported by
+// this version of the app API, but we don't want to break at the type checking level.
+// We only use this more permissive type for the `createApp` options, as we otherwise
+// want to stick to using the type for the outputs that we know about in this version
+// of the app api.
+type UnknownPluginOutput = {
+  type: string;
+};
+export type BackstagePluginWithAnyOutput = Omit<
+  BackstagePlugin<any, any>,
+  'output'
+> & {
+  output(): (PluginOutput | UnknownPluginOutput)[];
+};
+
 export type AppOptions = {
   /**
    * A collection of ApiFactories to register in the application to either
@@ -146,7 +162,7 @@ export type AppOptions = {
   /**
    * A list of all plugins to include in the app.
    */
-  plugins?: BackstagePlugin<any, any>[];
+  plugins?: BackstagePluginWithAnyOutput[];
 
   /**
    * Supply components to the app to override the default ones.