@@ -3184,6 +3184,7 @@ BREAKING CHANGE: Asset processing hooks in Compilation has been merged into a si
 			const buildInfo = module.buildInfo;
 			if (buildInfo) {
 				// create a new entry
+				/** @type {MemCache} */
 				const memCache = new WeakTupleMap();
 				moduleMemCacheCache.set(module, {
 					buildInfo,
 
@@ -756,6 +756,7 @@ module.exports = class MultiCompiler {
 		}
 		this.running = true;
 
+		/** @type {Watching[]} */
 		const watchings = this._runGraph(
 			(compiler, idx, callback, isBlocked, setChanged, setInvalid) => {
 				const watching = compiler.watch(
@@ -767,7 +768,8 @@ module.exports = class MultiCompiler {
 					watching._onChange = setChanged;
 					watching._isBlocked = isBlocked;
 				}
-				return watching;
+				// A child already running has none, and reported the error to `handler`.
+				return /** @type {Watching} */ (watching);
 			},
 			(compiler, /** @type {Watching} */ watching, _callback) => {
 				if (compiler.watching !== watching) return;
 
@@ -73,10 +73,12 @@ class MultiItemCache {
 		 * @returns {Promise<T | undefined>} promise with the data
 		 */
 		const next = (i) =>
-			this._items[i].getPromise().then((/** @type {T} */ result) => {
-				if (result !== undefined) return result;
-				if (++i < this._items.length) return next(i);
-			});
+			/** @type {Promise<T | undefined>} */ (this._items[i].getPromise()).then(
+				(result) => {
+					if (result !== undefined) return result;
+					if (++i < this._items.length) return next(i);
+				}
+			);
 		return /** @type {Promise<T>} */ (next(0));
 	}
 
 
@@ -555,7 +555,10 @@ const getObjectAndProperty = (config, schemaPath, index = 0) => {
 		if (isArray) {
 			if (value === undefined) {
 				value = {};
-				current[name] = [...Array.from({ length: index }), value];
+				current[name] = [
+					.../** @type {undefined[]} */ (Array.from({ length: index })),
+					value
+				];
 				cliAddedItems.set(current[name], index + 1);
 			} else if (!Array.isArray(value)) {
 				return {
@@ -613,15 +616,18 @@ const getObjectAndProperty = (config, schemaPath, index = 0) => {
 		const value = current[name];
 		if (value === undefined) {
 			/** @type {ConfigurationValue[]} */
-			const array = [...Array.from({ length: index }), undefined];
+			const array = [
+				.../** @type {undefined[]} */ (Array.from({ length: index })),
+				undefined
+			];
 			current[name] = array;
 			cliAddedItems.set(array, index + 1);
 			return { object: array, property: index, value: undefined };
 		} else if (!Array.isArray(value)) {
 			/** @type {ConfigurationValue[]} */
 			const array = [
 				/** @type {ConfigurationValue} */ (value),
-				...Array.from({ length: index }),
+				.../** @type {undefined[]} */ (Array.from({ length: index })),
 				undefined
 			];
 			current[name] = array;
 
@@ -9527,6 +9527,7 @@ const _evaluateCalcProduct = (tokens, cursor) => {
 		if (token.type === "/") {
 			const divisor = right.get("");
 			if (divisor === undefined || right.size !== 1) return null;
+			/** @type {Map<string, number>} */
 			const out = new Map();
 			for (const [key, coefficient] of sum) {
 				const quotient = foldDivide(coefficient, divisor);
@@ -9543,6 +9544,7 @@ const _evaluateCalcProduct = (tokens, cursor) => {
 		const factor = rightNumber !== undefined ? rightNumber : leftNumber;
 		const other = rightNumber !== undefined ? sum : right;
 		if (factor === undefined) return null;
+		/** @type {Map<string, number>} */
 		const out = new Map();
 		for (const [key, coefficient] of other) {
 			const product = foldMultiply(coefficient, factor);
@@ -10093,6 +10095,7 @@ const _neededPrefixes = (table, name) => {
 	/** @type {Set<string> | null} */
 	let result = null;
 	if (prefixes !== undefined) {
+		/** @type {Set<string>} */
 		const needed = new Set();
 		const prefixWindows = getPrefixWindows();
 		for (const [prefix, windows] of prefixes) {
@@ -16444,6 +16447,7 @@ const _propertiesOverlap = (a, b) => {
  * @returns {Set<string>} the property names, lowercased
  */
 const _blockProperties = (body) => {
+	/** @type {Set<string>} */
 	const out = new Set();
 	let depth = 0;
 	let quote = 0;
 
@@ -49,6 +49,7 @@ const processExportInfo = (
 		return;
 	}
 	// Only the recursive path needs the visited set; allocate it lazily
+	/** @type {Set<InstanceType<ExportInfo>>} */
 	const visited = alreadyVisited !== undefined ? alreadyVisited : new Set();
 	visited.add(exportInfo);
 	const exportsInfo = exportInfo.exportsInfo;
 
@@ -173,7 +173,9 @@ const INVALID = Symbol("invalid");
  * @property {boolean=} timestamp should use timestamp to snapshot
  */
 
-const DONE_ITERATOR_RESULT = new Set().keys().next();
+const DONE_ITERATOR_RESULT = /** @type {Set<never>} */ (new Set())
+	.keys()
+	.next();
 
 // cspell:word tshs
 // Tsh = Timestamp + Hash
@@ -1381,20 +1383,23 @@ class FileSystemInfo {
 			(s, v) => s.setManagedItemInfo(v),
 			false
 		);
+		/** @type {SnapshotOptimization<string, true>} */
 		this._managedFilesOptimization = new SnapshotOptimization(
 			(s) => s.hasManagedFiles(),
 			(s) => s.managedFiles,
 			(s, v) => s.setManagedFiles(v),
 			false,
 			true
 		);
+		/** @type {SnapshotOptimization<string, true>} */
 		this._managedContextsOptimization = new SnapshotOptimization(
 			(s) => s.hasManagedContexts(),
 			(s) => s.managedContexts,
 			(s, v) => s.setManagedContexts(v),
 			false,
 			true
 		);
+		/** @type {SnapshotOptimization<string, true>} */
 		this._managedMissingOptimization = new SnapshotOptimization(
 			(s) => s.hasManagedMissing(),
 			(s) => s.managedMissing,
 
@@ -385,6 +385,7 @@ const visitModules = (
 				stateEtag,
 				() => {
 					logger.time("visitModules: prepare");
+					/** @type {BlockModulesMap} */
 					const map = new Map();
 					const source = firstBlockModulesMapByModule.get(module);
 					if (source !== undefined) {
 
@@ -604,6 +604,7 @@ class HtmlGenerator extends Generator {
 		const crypto = require("crypto");
 
 		// Report each bad hash algorithm once, not once per referencing tag.
+		/** @type {Set<string>} */
 		const reportedBadAlgorithms = new Set();
 
 		const chunksById = getChunksById(compilation);
 
@@ -1624,6 +1624,7 @@ class HtmlModulesPlugin {
 				// long watch session can't accumulate stale entries.
 				compilation.hooks.afterSeal.tap(PLUGIN_NAME, () => {
 					if (sentinelResolvedSourceCache.size === 0) return;
+					/** @type {Set<string>} */
 					const live = new Set();
 					for (const chunk of compilation.chunks) {
 						live.add(String(chunk.id));
Original file line number	Diff line number	Diff line change
`@@ -49,6 +49,7 @@ const processExportInfo = (`
`49`	`49`	`return;`
`50`	`50`	`}`
`51`	`51`	`// Only the recursive path needs the visited set; allocate it lazily`
	`52`	`+ /** @type {Set<InstanceType<ExportInfo>>} */`
`52`	`53`	`const visited = alreadyVisited !== undefined ? alreadyVisited : new Set();`
`53`	`54`	`visited.add(exportInfo);`
`54`	`55`	`const exportsInfo = exportInfo.exportsInfo;`