Add regression test

ahejlsberg · ahejlsberg · commit 319ad71b159f · 2017-12-23T19:28:13.000-08:00
diff --git a/tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts b/tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts
@@ -0,0 +1,14 @@
+// @strict: true
+
+// Repro from #20840
+
+function function1<T extends 'a' | 'b'>(key: T) {
+  switch (key) {
+    case 'a':
+      key.toLowerCase();
+      break;
+    default:
+      key.toLowerCase();
+      break;
+  }
+}
