Add comments on warning limitations.

lib/mixins/element-mixin.js

@@ -778,8 +778,18 @@ export const ElementMixin = dedupingMixin(base => {
       // we don't want to spam existing users who might have adopted the
       // shorthand when attribute deserialization is not important.
       if (legacyOptimizations && !(prop in this._properties) &&
+          // Methods used in templates with no dependencies (or only literal
+          // dependencies) become accessors with template effects; ignore these 
           !(effect.info.part.signature && effect.info.part.signature.static) &&
-          !effect.info.part.hostProp && !templateInfo.nestedTemplate) {
+          // Bindings added from a host to the template element may also include
+          // "instance props" introduced into the template scope and not
+          // actually provided by the host; since we can't disambiguate these,
+          // ignore them all
+          !effect.info.part.hostProp &&
+          // Template effects added on TemplateInstance classes can't determine
+          // whether the property may have been a "static" method, so ignore
+          // all nested template bindings
+          !templateInfo.nestedTemplate) {
         console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
           `attribute will not be observed.`);
       }