Add regression test

tests/cases/conformance/types/mapped/recursiveMappedTypes.ts

@@ -61,3 +61,21 @@ type Remap2<T> = T extends object ? { [P in keyof T]: Remap2<T[P]>; } : T;
   
 type a = Remap1<string[]>;  // string[]
 type b = Remap2<string[]>;  // string[]
+
+// Repro from #29992
+
+type NonOptionalKeys<T> = { [P in keyof T]: undefined extends T[P] ? never : P }[keyof T];
+type Child<T> = { [P in NonOptionalKeys<T>]: T[P] }
+
+export interface ListWidget {
+    "type": "list",
+    "minimum_count": number,
+    "maximum_count": number,
+    "collapsable"?: boolean, //default to false, means all expanded
+    "each": Child<ListWidget>;
+}
+
+type ListChild = Child<ListWidget>
+
+declare let x: ListChild;
+x.type;