Add regression test

Author: ahejlsberg

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

@@ -527,3 +527,30 @@ class Form<T> {
         this.childFormFactories[prop](value)
     }
 }
+
+// Repro from #13787
+
+class SampleClass<P> {
+    public props: Readonly<P>;
+    constructor(props: P) {
+        this.props = Object.freeze(props);
+    }
+}
+
+interface Foo {
+    foo: string;
+}
+
+declare function merge<T, U>(obj1: T, obj2: U): T & U;
+
+class AnotherSampleClass<T> extends SampleClass<T & Foo> {
+    constructor(props: T) {
+        const foo: Foo = { foo: "bar" };
+        super(merge(props, foo));
+    }
+
+    public brokenMethod() {
+        this.props.foo.concat;
+    }
+}
+new AnotherSampleClass({});