Add failing repro

Author: ahejlsberg

tests/cases/conformance/controlFlow/typeGuardsTypeParameters.ts

@@ -0,0 +1,35 @@
+// @strictNullChecks: true
+
+// Type guards involving type parameters produce intersection types
+
+class C {
+    prop: string;
+}
+
+function f1<T>(x: T) {
+    if (x instanceof C) {
+        let v1: T = x;
+        let v2: C = x;
+        x.prop;
+    }
+}
+
+function f2<T>(x: T) {
+    if (typeof x === "string") {
+        let v1: T = x;
+        let v2: string = x;
+        x.length;
+    }
+}
+
+// Repro from #13872
+
+function fun<T>(item: { [P in keyof T]: T[P] }) {
+    const strings: string[] = [];
+    for (const key in item) {
+        const value = item[key];
+        if (typeof value === "string") {
+            strings.push(value);
+        }
+    }
+}