Warn when setting a property that has only a zero-arg getter. (#202)

can-define.js

@@ -253,11 +253,21 @@ define.property = function(objPrototype, prop, definition, dataInitializers, com
 		// Add `set` functionality to the eventSetter.
 		setter = make.set.setter(prop, definition.set, reader, eventsSetter, false);
 	}
-	// If there's niether `set` or `get`,
+	// If there's neither `set` or `get`,
 	else if (!definition.get) {
 		// make a set that produces events.
 		setter = eventsSetter;
 	}
+	//!steal-remove-start
+	// If there's zero-arg `get` but not `set`, warn on all sets in dev mode
+	else if (definition.get.length < 1) {
+		setter = function() {
+			dev.warn("Set value for property " +
+				prop +
+				" ignored, as its definition has a zero-argument getter and no setter");
+		};
+	}
+	//!steal-remove-end
 
 	// Add type behavior to the setter.
 	if (type) {
@@ -326,6 +336,7 @@ make = {
 				handler: function(newVal) {
 					var oldValue = computeObj.oldValue;
 					computeObj.oldValue = newVal;
+
 					canEvent.dispatch.call(map, {
 						type: prop,
 						target: map,

define-test.js

@@ -5,6 +5,7 @@ var CanList = require("can-define/list/list");
 var canBatch = require("can-event/batch/batch");
 var each = require("can-util/js/each/each");
 var canSymbol = require("can-symbol");
+var canDev = require("can-util/js/dev/dev");
 
 QUnit.module("can-define");
 
@@ -1368,3 +1369,32 @@ QUnit.test('define() should add a CID (#246)', function() {
 	var g = new Greeting();
 	QUnit.ok(g._cid, "should have a CID property");
 });
+
+if(System.env.indexOf("production") < 0) {
+	QUnit.test('Setting a value with only a get() generates a warning (#202)', function() {
+		QUnit.expect(2);
+		var VM = function() {};
+		define(VM.prototype, {
+			derivedProp: {
+				get: function() {
+					return "Hello World";
+				}
+			}
+		});
+
+		var vm = new VM();
+		vm.on("derivedProp", function() {});
+
+		var oldwarn = canDev.warn;
+		canDev.warn = function(mesg) {
+			QUnit.equal(
+				mesg,
+				"Set value for property derivedProp ignored, as its definition has a zero-argument getter and no setter",
+				"Warning is expected message");
+		};
+
+		vm.derivedProp = 'prop is set';
+		QUnit.equal(vm.derivedProp, "Hello World", "Getter value is preserved");
+		canDev.warn = oldwarn;
+	});
+}