add test for #12536

Author: wonderful-panda

tests/cases/fourslash/completionForStringLiteral5.ts

@@ -0,0 +1,15 @@
+/// <reference path='fourslash.ts'/>
+
+////interface Foo {
+////    foo: string;
+////    bar: string;
+////}
+////
+////function f<K extends keyof Foo>(a: K) { };
+////f("/*1*/
+
+goTo.marker('1');
+verify.completionListContains("foo");
+verify.completionListContains("bar");
+verify.memberListCount(2);
+