add extra test case

Author: Andarist

tests/cases/fourslash/quickInfoMappedPropertyUnionUndefined4.ts

@@ -0,0 +1,11 @@
+// @strict: true
+
+//// type A/*1*/ = { [K in keyof { a?: string }]-?: string }
+//// type B/*2*/ = { [K in keyof A]: string | undefined }
+
+verify.quickInfoAt("1", `type A = {
+    a: string;
+}`);
+verify.quickInfoAt("2", `type B = {
+    a: string | undefined;
+}`);