Legacy warnings 3.x

lib/mixins/element-mixin.js

@@ -195,7 +195,6 @@ export const ElementMixin = dedupingMixin(base => {
    * disables the effect, the setter would fail unexpectedly.
    * Based on feedback, we may want to try to make effects more malleable
    * and/or provide an advanced api for manipulating them.
-   * Also consider adding warnings when an effect cannot be changed.
    *
    * @param {!PolymerElement} proto Element class prototype to add accessors
    *   and effects to
@@ -217,17 +216,27 @@ export const ElementMixin = dedupingMixin(base => {
     // setup where multiple triggers for setting a property)
     // While we do have `hasComputedEffect` this is set on the property's
     // dependencies rather than itself.
-    if (info.computed && !proto._hasReadOnlyEffect(name)) {
-      proto._createComputedProperty(name, info.computed, allProps);
+    if (info.computed) {
+      if (proto._hasReadOnlyEffect(name)) {
+        console.warn(`Cannot redefine computed property '${name}'.`);
+      } else {
+        proto._createComputedProperty(name, info.computed, allProps);
+      }
     }
     if (info.readOnly && !proto._hasReadOnlyEffect(name)) {
       proto._createReadOnlyProperty(name, !info.computed);
+    } else if (info.readOnly === false && proto._hasReadOnlyEffect(name)) {
+      console.warn(`Cannot make readOnly property '${name}' non-readOnly.`);
     }
     if (info.reflectToAttribute && !proto._hasReflectEffect(name)) {
       proto._createReflectedProperty(name);
+    } else if (info.reflectToAttribute === false && proto._hasReflectEffect(name)) {
+      console.warn(`Cannot make reflected property '${name}' non-reflected.`);
     }
     if (info.notify && !proto._hasNotifyEffect(name)) {
       proto._createNotifyingProperty(name);
+    } else if (info.notify === false && proto._hasNotifyEffect(name)) {
+      console.warn(`Cannot make notify property '${name}' non-notify.`);
     }
     // always add observer
     if (info.observer) {

lib/mixins/property-effects.js

@@ -18,7 +18,7 @@ import { camelToDashCase, dashToCamelCase } from '../utils/case-map.js';
 import { PropertyAccessors } from './property-accessors.js';
 /* for annotated effects */
 import { TemplateStamp } from './template-stamp.js';
-import { sanitizeDOMValue } from '../utils/settings.js';
+import { sanitizeDOMValue, legacyOptimizations } from '../utils/settings.js';
 
 // Monotonically increasing unique ID used for de-duping effects triggered
 // from multiple properties in the same turn
@@ -2390,6 +2390,15 @@ export const PropertyEffects = dedupingMixin(superClass => {
      * @protected
      */
     static _addTemplatePropertyEffect(templateInfo, prop, effect) {
+      // `dynamicFns` is the flattened property list, so we can use that to
+      // detect non-declared properties.  Properties must be listed in
+      // `properties` to be included in `observedAttributes` since CE V1
+      // reads that at registration time, and we want to keep template parsing
+      // lazy
+      if (legacyOptimizations && !(prop in templateInfo.dynamicFns)) {
+        console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
+          `attribute will not be observed.`);
+      }
       let hostProps = templateInfo.hostProps = templateInfo.hostProps || {};
       hostProps[prop] = true;
       let effects = templateInfo.propertyEffects = templateInfo.propertyEffects || {};