Add ancestor polyfill during block registration

packages/blocks/src/api/registration.js

@@ -164,6 +164,17 @@ export function unstable__bootstrapServerSideBlockDefinitions( definitions ) {
 				serverSideBlockDefinitions[ blockName ].apiVersion =
 					definitions[ blockName ].apiVersion;
 			}
+			// The `ancestor` prop is not included in the definitions shared
+			// from the server yet, so it needs to be polyfilled as well.
+			// @see https://github.com/WordPress/gutenberg/pull/39894
+			if (
+				serverSideBlockDefinitions[ blockName ].ancestor ===
+					undefined &&
+				definitions[ blockName ].ancestor
+			) {
+				serverSideBlockDefinitions[ blockName ].ancestor =
+					definitions[ blockName ].ancestor;
+			}
 			continue;
 		}
 		serverSideBlockDefinitions[ blockName ] = mapKeys(
@@ -187,6 +198,7 @@ function getBlockSettingsFromMetadata( { textdomain, ...metadata } ) {
 		'title',
 		'category',
 		'parent',
+		'ancestor',
 		'icon',
 		'description',
 		'keywords',

packages/blocks/src/api/test/registration.js

@@ -415,6 +415,42 @@ describe( 'blocks', () => {
 			} );
 		} );
 
+		// This test can be removed once the polyfill for ancestor gets removed.
+		it( 'should apply ancestor on the client when not set on the server', () => {
+			const blockName = 'core/test-block-with-ancestor';
+			unstable__bootstrapServerSideBlockDefinitions( {
+				[ blockName ]: {
+					category: 'widgets',
+				},
+			} );
+			unstable__bootstrapServerSideBlockDefinitions( {
+				[ blockName ]: {
+					ancestor: 'core/test-block-ancestor',
+					category: 'ignored',
+				},
+			} );
+
+			const blockType = {
+				title: 'block title',
+			};
+			registerBlockType( blockName, blockType );
+			expect( getBlockType( blockName ) ).toEqual( {
+				ancestor: 'core/test-block-ancestor',
+				name: blockName,
+				save: expect.any( Function ),
+				title: 'block title',
+				category: 'widgets',
+				icon: { src: BLOCK_ICON_DEFAULT },
+				attributes: {},
+				providesContext: {},
+				usesContext: [],
+				keywords: [],
+				supports: {},
+				styles: [],
+				variations: [],
+			} );
+		} );
+
 		it( 'should validate the icon', () => {
 			const blockType = {
 				save: noop,