fix(core): always treat error boundary fallback as a callback

packages/docusaurus-module-type-aliases/src/index.d.ts

@@ -80,9 +80,9 @@ declare module '@theme/Error' {
   import type {ComponentProps} from 'react';
   import type ErrorBoundary from '@docusaurus/ErrorBoundary';
 
-  type ErrorProps = ComponentProps<
+  type ErrorProps = Parameters<
     NonNullable<ComponentProps<typeof ErrorBoundary>['fallback']>
-  >;
+  >[0];
 
   export interface Props extends ErrorProps {}
   export default function Error(props: Props): JSX.Element;
@@ -125,13 +125,13 @@ declare module '@docusaurus/constants' {
 }
 
 declare module '@docusaurus/ErrorBoundary' {
-  import type {ReactNode, ComponentType} from 'react';
+  import type {ReactNode} from 'react';
 
   export interface Props {
-    readonly fallback?: ComponentType<{
+    readonly fallback?: (props: {
       readonly error: Error;
       readonly tryAgain: () => void;
-    }>;
+    }) => JSX.Element;
     readonly children: ReactNode;
   }
   export default function ErrorBoundary(props: Props): JSX.Element;

packages/docusaurus/src/client/exports/ErrorBoundary.tsx

@@ -32,10 +32,12 @@ export default class ErrorBoundary extends React.Component<Props, State> {
     const {error} = this.state;
 
     if (error) {
-      const Fallback = this.props.fallback ?? DefaultFallback;
-      return (
-        <Fallback error={error} tryAgain={() => this.setState({error: null})} />
-      );
+      const fallback =
+        this.props.fallback ?? ((props) => <DefaultFallback {...props} />);
+      return fallback({
+        error,
+        tryAgain: () => this.setState({error: null}),
+      });
     }
 
     // See https://github.com/facebook/docusaurus/issues/6337#issuecomment-1012913647

website/docs/docusaurus-core.md

@@ -55,7 +55,7 @@ This component doesn't catch build-time errors and only protects against client-
 
 #### Props {#errorboundary-props}
 
-- `fallback`: a React component. The error boundary will render the component with two props: `error`, the error that was caught, and `tryAgain`, a function (`() => void`) callback to reset the error in the component and try rendering it again.
+- `fallback`: an optional callback returning a JSX element. It will receive two props: `error`, the error that was caught, and `tryAgain`, a function (`() => void`) callback to reset the error in the component and try rendering it again. If not present, `@theme/Error` will be rendered instead.
 
 ### `<Head/>` {#head}