Add tests

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

@@ -0,0 +1,36 @@
+// @strict: true
+
+// Repro from #20138
+
+class C { }
+
+function f1<T extends C>(v: T | string): void {
+    if (v instanceof C) {
+        const x: T = v;
+    }
+    else {
+        const s: string = v;
+    }
+}
+
+class D { }
+
+function f2<T extends C, U extends D>(v: T | U) {
+    if (v instanceof C) {
+        const x: T = v;
+    }
+    else {
+        const y: U = v;
+    }
+}
+
+class E { x: string }
+
+function f3<T extends E>(v: T | { x: string }) {
+    if (v instanceof E) {
+        const x: T = v;
+    }
+    else {
+        const y: { x: string } = v;
+    }
+}