Support custom Error subclass in the TypeScript types (#13)

Co-authored-by: Sindre Sorhus <[email protected]>

Author: gr2m

index.d.ts

@@ -1,7 +1,7 @@
 /**
 Create an error from multiple errors.
 */
-declare class AggregateError extends Error implements Iterable<Error> {
+declare class AggregateError<T extends Error = Error> extends Error implements Iterable<T> {
 	readonly name: 'AggregateError';
 
 	/**
@@ -43,9 +43,9 @@ declare class AggregateError extends Error implements Iterable<Error> {
 	//=> [Error: baz]
 	```
 	*/
-	constructor(errors: ReadonlyArray<Error | {[key: string]: any} | string>);
+	constructor(errors: ReadonlyArray<T | {[key: string]: any} | string>);
 
-	[Symbol.iterator](): IterableIterator<Error>;
+	[Symbol.iterator](): IterableIterator<T>;
 }
 
 export = AggregateError;

index.test-d.ts

@@ -12,3 +12,20 @@ expectType<IterableIterator<Error>>(aggregateError[Symbol.iterator]());
 for (const error of aggregateError) {
 	expectType<Error>(error);
 }
+
+class CustomError extends Error {
+	public foo: string;
+
+	constructor(message: string) {
+		super(message)
+		this.name = 'CustomError';
+		this.foo = 'bar';
+	}
+}
+const customAggregateError = new AggregateError<CustomError>([
+	new CustomError('foo')
+]);
+
+for (const error of customAggregateError) {
+	expectType<string>(error.foo);
+}