Add regression test

Author: ahejlsberg

tests/cases/conformance/types/literal/literalTypeWidening.ts

@@ -127,3 +127,10 @@ export type LangCode = keyof typeof langCodeSet
 export const langCodes = keys(langCodeSet)
 
 const arr: Obj[] = langCodes.map(code => ({ code }))
+
+// Repro from #29081
+
+function test<T extends { a: string, b: string }>(obj: T): T {
+    let { a, ...rest } = obj;
+    return { a: 'hello', ...rest } as T;
+}