RFC: Getter CP readOnly by default

0000-read-only-cp-syntax.md

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+- Start Date: 2014-19-30
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Summary
+
+Getter only computed properties should be readOnly by default
+
+# Motivation
+
+Overridable CP's are unexpected, hard to learn, and wanted in the minority of
+cases. When it happens unexpeditly it is nearly always unwanted and the resulting
+chaos is hard to debug.
+
+
+```js
+var User = Ember.Object.extend({
+  isOld: Ember.computed('age', function() {
+     return this.get('age') > 30;
+  });
+});
+
+var user = User.create({
+  age: 30
+});
+
+user.get('isOld'); // => false
+
+user.set('age', 31);
+user.get('isOld'); // => true
+
+user.set('isOld', false);
+user.get('isOld'); // => false
+
+user.set('age', 30);
+user.get('isOld'); // => false (EWUT?)
+```
+
+
+What just happened:
+
+* although age > 30 : isOld remains false
+* by explicitly setting isOld, we have diverged the CP to no longer invalidate based on its dependentKey.
+* many bugs and hair loss
+* no way to sensibly recover
+* rarely wanted
+
+
+# Detailed design
+
+```js
+firstName: Ember.computed(function() {
+
+}); // readOnly
+```
+
+Is equivalent to:
+
+```js
+firstName: Ember.computed({
+  get: function() {
+}); // readOnly (same as above)
+```
+
+---
+
+```js
+firstName: Ember.computed(function() {
+
+}).overridable(); // writable
+```
+
+Is equivalent to:
+
+```js
+firstName: Ember.computed({
+  get: function() {
+}).overridable(); // writable
+```
+
+---
+
+
+```js
+firstName: Ember.computed(function(key, value) {
+
+}).overridable(); // throw new Error("Overridable cannot be used if a setter already exists....");
+```
+
+Is equivalent to:
+
+```js
+firstName: Ember.computed({
+  get: function() { },
+  set: function() { }
+}).overridable(); // throw new Error("Overridable cannot be used if a setter already exists....");
+```
+
+---
+
+```js
+firstName: Ember.computed({
+  get: function() { },
+  set: function() { }
+}).readOnly();    // throw new Error("Cannot mark a CP as readOnly if it has an explicit setter");
+```
+
+```js
+
+firstName: Ember.computed({
+  get: function() { }
+}); // on super class
+
+firstName: Ember.computed({
+  get: function() { },
+  set: function(key, value) { this._super(key, value); }
+}); // throw new Error("ReadOnly Error blah blah blah");
+```
+
+# migration:
+
+* 1.x update internals to correctly use `writable` if needed (and runs tests against this)
+* 1.x deprecate writability unless `writable` was explicitly called, or new `set` syntax  was used
+* 2.0 flip the switch to `readOnly` by default and never look back.
+
+# Drawbacks
+
+N/A
+
+# Alternatives
+
+N/A
+
+# Unresolved questions
+
+N/A