Better typings for Promise.then(), like microsoft#31117

src/lib/es5.d.ts

@@ -1370,7 +1370,7 @@ interface PromiseLike<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T extends PromiseLike<infer U> ? U : T) => TResult1) | undefined | null, onrejected?: ((reason: any) => TResult2) | undefined | null): PromiseLike<TResult1 | TResult2>;
 }
 
 /**
@@ -1383,14 +1383,14 @@ interface Promise<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T extends PromiseLike<infer U> ? U : T) => TResult1) | undefined | null, onrejected?: ((reason: any) => TResult2) | undefined | null): Promise<TResult1 | TResult2>;
 
     /**
      * Attaches a callback for only the rejection of the Promise.
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of the callback.
      */
-    catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult>;
+    catch<TResult = never>(onrejected?: ((reason: any) => TResult) | undefined | null): Promise<T | TResult>;
 }
 
 interface ArrayLike<T> {