Added a test.

tests/cases/conformance/types/typeRelationships/instanceOf/narrowingGenericTypeFromInstanceof01.ts

@@ -0,0 +1,27 @@
+class A<T> {
+    constructor(private a: string) { }
+}
+
+class B<T> {
+}
+
+function acceptA<T>(a: A<T>) { }
+function acceptB<T>(b: B<T>) { }
+
+function test<T>(x: A<T> | B<T>) {
+    if (x instanceof B) {
+        acceptA(x);
+    }
+
+    if (x instanceof A) {
+        acceptA(x);
+    }
+
+    if (x instanceof B) {
+        acceptB(x);
+    }
+
+    if (x instanceof B) {
+        acceptB(x);
+    }
+}