Add tests

Author: ahejlsberg

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

@@ -170,3 +170,15 @@ function f76<T, U extends T, K extends keyof T>(x: { [P in K]: T[P] }, y: { [P i
     x = y;
     y = x;  // Error
 }
+
+function f80<T>(t: T): Partial<T> {
+    return t;
+}
+
+function f81<T, K extends keyof T>(t: T, k: K): Partial<T[K]> {
+    return t[k];
+}
+
+function f82<T, K1 extends keyof T, K2 extends keyof T[K1]>(t: T, k1: K1, k2: K2): Partial<T[K1][K2]> {
+    return t[k1][k2];
+}