[BUGFIX LTS] Don't run getters while applying mixins

This change ensures that getters are never evaluated while applying
mixins.

It relies on the fact that all getters (including undecorated ones) get
converted into classic decorators when the mixin is originally created.

packages/@ember/-internals/metal/lib/mixin.ts

@@ -28,6 +28,7 @@ import {
   ComputedDescriptor,
   descriptorForDecorator,
   descriptorForProperty,
+  isClassicDecorator,
   makeComputedDecorator,
   nativeDescDecorator,
 } from './decorator';
@@ -237,7 +238,7 @@ function mergeMixins(
   keys: string[],
   keysWithSuper: string[]
 ): void {
-  let currentMixin;
+  let currentMixin: MixinLike | undefined;
 
   for (let i = 0; i < mixins.length; i++) {
     currentMixin = mixins[i];
@@ -304,12 +305,17 @@ function mergeProps(
       let desc = meta.peekDescriptors(key);
 
       if (desc === undefined) {
-        // The superclass did not have a CP, which means it may have
-        // observers or listeners on that property.
-        let prev = (values[key] = base[key]);
-
-        if (typeof prev === 'function') {
-          updateObserversAndListeners(base, key, prev, false);
+        // If the value is a classic decorator, we don't want to actually
+        // access it, because that will execute the decorator while we're
+        // building the class.
+        if (!isClassicDecorator(value)) {
+          // The superclass did not have a CP, which means it may have
+          // observers or listeners on that property.
+          let prev = (values[key] = base[key]);
+
+          if (typeof prev === 'function') {
+            updateObserversAndListeners(base, key, prev, false);
+          }
         }
       } else {
         descs[key] = desc;

packages/@ember/-internals/runtime/tests/mixins/accessor_test.js

@@ -0,0 +1,56 @@
+import EmberObject from '@ember/object';
+import { moduleFor, RenderingTestCase } from 'internal-test-helpers';
+
+moduleFor(
+  'runtime: Mixin Accessors',
+  class extends RenderingTestCase {
+    ['@test works with getters'](assert) {
+      let value = 'building';
+
+      let Base = EmberObject.extend({
+        get foo() {
+          if (value === 'building') {
+            throw Error('base should not be called yet');
+          }
+
+          return "base's foo";
+        },
+      });
+
+      // force Base to be finalized so its properties will contain `foo`
+      Base.proto();
+
+      class Child extends Base {
+        get foo() {
+          if (value === 'building') {
+            throw Error('child should not be called yet');
+          }
+
+          return "child's foo";
+        }
+      }
+
+      Child.proto();
+
+      let Grandchild = Child.extend({
+        get foo() {
+          if (value === 'building') {
+            throw Error('grandchild should not be called yet');
+          }
+
+          return value;
+        },
+      });
+
+      let instance = Grandchild.create();
+
+      value = 'done building';
+
+      assert.equal(instance.foo, 'done building', 'getter defined correctly');
+
+      value = 'changed value';
+
+      assert.equal(instance.foo, 'changed value', 'the value is a real getter, not a snapshot');
+    }
+  }
+);