Add tests

Author: ahejlsberg

tests/cases/conformance/types/union/discriminatedUnionTypes2.ts

@@ -72,3 +72,25 @@ function f20<Data>(carrier: DataCarrier<Data>) {
         const data: null = carrier.data
     }
 }
+
+// Repro from #28935
+
+type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+
+function f30(foo: Foo) {
+    if (foo.tag) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
+
+function f31(foo: Foo) {
+    if (foo.tag === true) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}