Add regression test

tests/cases/compiler/deeplyNestedCheck.ts

@@ -0,0 +1,9 @@
+// Repro from #14794
+
+interface DataSnapshot<X = {}> {
+  child(path: string): DataSnapshot;
+}
+
+interface Snapshot<T> extends DataSnapshot {
+  child<U extends keyof T>(path: U): Snapshot<T[U]>;
+}