fix: fix marked types (#3103)

src/marked.ts

@@ -26,13 +26,13 @@ const markedInstance = new Marked();
 export function marked(src: string, options: MarkedOptions & { async: true }): Promise<string>;
 
 /**
- * Compiles markdown to HTML synchronously.
+ * Compiles markdown to HTML.
  *
  * @param src String of markdown source to be compiled
  * @param options Optional hash of options
- * @return String of compiled HTML
+ * @return String of compiled HTML. Wil be a Promise of string if async is set to true by any extensions.
  */
-export function marked(src: string, options?: MarkedOptions): string;
+export function marked(src: string, options?: MarkedOptions): string | Promise<string>;
 export function marked(src: string, opt?: MarkedOptions): string | Promise<string> {
   return markedInstance.parse(src, opt);
 }

test/types/marked.ts

@@ -247,13 +247,21 @@ marked.use(asyncExtension);
 const md = '# foobar';
 const asyncMarked: string = await marked(md, { async: true });
 const promiseMarked: Promise<string> = marked(md, { async: true });
+// @ts-expect-error marked can still be async if an extension sets `async: true`
 const notAsyncMarked: string = marked(md, { async: false });
+// @ts-expect-error marked can still be async if an extension sets `async: true`
 const defaultMarked: string = marked(md);
+// as string can be used if no extensions set `async: true`
+const stringMarked: string = marked(md) as string;
 
 const asyncMarkedParse: string = await marked.parse(md, { async: true });
 const promiseMarkedParse: Promise<string> = marked.parse(md, { async: true });
+// @ts-expect-error marked can still be async if an extension sets `async: true`
 const notAsyncMarkedParse: string = marked.parse(md, { async: false });
+// @ts-expect-error marked can still be async if an extension sets `async: true`
 const defaultMarkedParse: string = marked.parse(md);
+// as string can be used if no extensions set `async: true`
+const stringMarkedParse: string = marked.parse(md) as string;
 })();
 
 // Tests for List and ListItem