Initial investigation of exhaustiveness checking issue

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

test-issue.ts

@@ -0,0 +1,49 @@
+// Test the issue: Exhaustiveness checking against an enum with 1 member
+
+enum ActionTypes {
+  INCREMENT = 'INCREMENT',
+}
+
+interface IIncrement {
+  payload: {};
+  type: ActionTypes.INCREMENT;
+}
+
+type AnyStringExcept<T extends string> = { [P in T]: never; };
+
+type ValidAction = IIncrement;
+type UnhandledAction = { type: AnyStringExcept<ActionTypes>; };
+type PossibleAction = ValidAction | UnhandledAction;
+
+function isUnhandled(x: PossibleAction): x is UnhandledAction {
+    return !(x.type in ActionTypes);
+}
+
+type CounterState = number;
+const initialState: CounterState = 0;
+
+function receiveAction(state = initialState, action: PossibleAction) {
+    if (isUnhandled(action)) {
+        return state;
+    }
+
+    // typeof action === ValidAction
+    switch (action.type) {
+        case ActionTypes.INCREMENT:
+            return state + 1;
+    }
+
+    // This should not error - all cases are handled
+    const n: never = action;
+    return state;
+}
+
+// Simpler test case from RyanCavanaugh's comment
+function fn(obj: { name: "bob" }) {
+  if (obj.name == "bob") {
+    // bob case
+  } else {
+    // Should not be an error
+    const n: never = obj;
+  }
+}