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

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');
+    }
+  }
+);

packages/@ember/object/mixin.ts

@@ -8,12 +8,13 @@ import { guidFor, observerListenerMetaFor, ROOT, wrap } from '@ember/-internals/
 import { assert } from '@ember/debug';
 import { DEBUG } from '@glimmer/env';
 import { _WeakSet } from '@glimmer/util';
-import type {
-  ComputedDecorator,
-  ComputedPropertyGetter,
-  ComputedPropertyObj,
-  ComputedPropertySetter,
-  ComputedDescriptor,
+import {
+  type ComputedDecorator,
+  type ComputedPropertyGetter,
+  type ComputedPropertyObj,
+  type ComputedPropertySetter,
+  type ComputedDescriptor,
+  isClassicDecorator,
 } from '@ember/-internals/metal';
 import {
   ComputedProperty,
@@ -235,7 +236,7 @@ function mergeMixins(
   keys: string[],
   keysWithSuper: string[]
 ): void {
-  let currentMixin;
+  let currentMixin: MixinLike | undefined;
 
   for (let i = 0; i < mixins.length; i++) {
     currentMixin = mixins[i];
@@ -309,12 +310,17 @@ 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;