Add regression test

Author: ahejlsberg

tests/cases/compiler/controlFlowDestructuringLoop.ts

@@ -0,0 +1,24 @@
+// @strict: true
+
+// Repro from #28758
+
+interface NumVal { val: number; }
+interface StrVal { val: string; }
+type Val = NumVal | StrVal;
+
+function isNumVal(x: Val): x is NumVal {
+    return typeof x.val === 'number';
+}
+
+function foo(things: Val[]): void {
+    for (const thing of things) {
+        if (isNumVal(thing)) {
+            const { val } = thing;
+            val.toFixed(2);
+        }
+        else {
+            const { val } = thing;
+            val.length;
+        }
+    }
+}