Add tests

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

@@ -13,6 +13,10 @@ function f2<T>(x: Partial<T>, y: Readonly<T>) {
     obj = y;
 }
 
+function f3<T>(x: Partial<T>) {
+    x = {};
+}
+
 // Repro from #12900
 
 interface Base {
@@ -33,3 +37,9 @@ interface E2 extends Base {
 interface E3<T> extends Base {
     foo: Partial<T>; // or other mapped type
 }
+
+// Repro from #13747
+
+class Form<T> {
+    private values: {[P in keyof T]?: T[P]} = {}
+}