Add test

tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts

@@ -0,0 +1,69 @@
+// Repro from #7271
+
+class C1 { item: string }
+class C2 { item: string[] }
+class C3 { item: string }
+
+function foo1(x: C1 | C2 | C3): string {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+
+function foo2(x: C1 | C2 | C3): string {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+
+// More tests
+
+class A { a: string }
+class A1 extends A { }
+class A2 { a: string }
+class B extends A { b: string }
+
+function goo(x: A) {
+    if (x instanceof A) {
+        x;  // A
+    }
+    else {
+        x;  // never
+    }
+    if (x instanceof A1) {
+        x;  // A1
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof A2) {
+        x;  // A2
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof B) {
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+}