Add tests

Author: ahejlsberg

tests/cases/conformance/types/keyof/keyofAndIndexedAccess.ts

@@ -302,23 +302,16 @@ type S2 = {
     b: string;
 };
 
-function f90<T extends S2, K extends keyof S2>(x1: S2[keyof S2], x2: T[keyof S2], x3: S2[K], x4: T[K]) {
+function f90<T extends S2, K extends keyof S2>(x1: S2[keyof S2], x2: T[keyof S2], x3: S2[K]) {
     x1 = x2;
     x1 = x3;
-    x1 = x4;
     x2 = x1;
     x2 = x3;
-    x2 = x4;
     x3 = x1;
     x3 = x2;
-    x3 = x4;
-    x4 = x1;
-    x4 = x2;
-    x4 = x3;
     x1.length;
     x2.length;
     x3.length;
-    x4.length;
 }
 
 function f91<T, K extends keyof T>(x: T, y: T[keyof T], z: T[K]) {

tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts

@@ -122,3 +122,22 @@ function f4<T extends { [K in keyof T]: string }>(k: keyof T) {
     k = 42; // error
     k = "hello"; // error
 }
+
+// Repro from #27470
+
+type UndefinedKeys<T extends Record<string, any>> = {
+  [K in keyof T]: undefined extends T[K] ? K : never
+};
+
+type MyType = {a: string, b: string | undefined}
+
+type Result1 = UndefinedKeys<MyType>;
+
+const a1: Result1['a'] = 'a';  // Error
+const b1: Result1['b'] = 'b';
+
+function test1<T extends Record<string, any>, K extends keyof T>(t: T, k: K) {
+    t[k] = 42;  // Error
+    t[k] = "hello";  // Error
+    t[k] = [10, 20];  // Error
+}