Fix: dynamic modifier tag not cleared when instance becomes undefined

packages/@glimmer-workspace/integration-tests/test/modifiers/dynamic-modifiers-test.ts

@@ -87,6 +87,52 @@ class DynamicModifiersResolutionModeTest extends RenderTest {
     assert.deepEqual(receivedArgs, ['y', 'x']);
   }
 
+  @test
+  'Dynamic modifier becoming undefined does not error when args change afterwards'(assert: Assert) {
+    let installCount = 0;
+    let lastValue: unknown;
+
+    // The modifier reads args[0], so its tag tracks the arg's reactivity.
+    // When outer changes and the modifier's tag is invalidated, the bug triggers:
+    // scheduleUpdateModifier would be called with undefined instance.
+    const foo = defineSimpleModifier((element: Element, args: unknown[]) => {
+      installCount++;
+      lastValue = args[0];
+      (element as HTMLElement).dataset['value'] = String(args[0]);
+    });
+
+    this.render(
+      `
+      {{~#let (modifier this.foo this.outer) as |bar|~}}
+        <div {{ (if this.cond bar) }}>content</div>
+      {{~/let~}}
+      `,
+      { foo, outer: 'initial', cond: true }
+    );
+
+    this.assertHTML('<div data-value="initial">content</div>');
+    assert.strictEqual(installCount, 1, 'modifier installed once on initial render');
+    assert.strictEqual(lastValue, 'initial');
+
+    // Disable the modifier — newInstance becomes undefined, instance is destroyed
+    // (data-value persists because no cleanup function was returned)
+    this.rerender({ cond: false });
+    this.assertHTML('<div data-value="initial">content</div>');
+
+    // Change args while the modifier is disabled — this triggered the bug:
+    // "Cannot destructure property 'manager' of '.for' as it is undefined"
+    // because the modifier's tag was invalidated by the arg change, and the
+    // stale tag caused scheduleUpdateModifier to be called with undefined.
+    this.rerender({ outer: 'changed' });
+    this.assertHTML('<div data-value="initial">content</div>');
+
+    // Re-enable the modifier — should install cleanly with updated args
+    this.rerender({ cond: true });
+    this.assertHTML('<div data-value="changed">content</div>');
+    assert.strictEqual(installCount, 2, 'modifier installed again after re-enabling');
+    assert.strictEqual(lastValue, 'changed');
+  }
+
   @test
   'Can pass curried modifier as argument and invoke dynamically (with args)'() {
     const foo = defineSimpleModifier(

packages/@glimmer/runtime/lib/compiled/opcodes/dom.ts

@@ -363,11 +363,16 @@ export class UpdateDynamicModifierOpcode implements UpdatingOpcode {
 
         this.tag = tag;
         vm.env.scheduleInstallModifier(newInstance);
+      } else {
+        tag = null;
+        this.tag = null;
       }
 
       this.instance = newInstance;
     } else if (tag !== null && !validateTag(tag, lastUpdated)) {
-      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion -- @fixme
+      // tag !== null guarantees instance is defined: this.tag is only set to a
+      // non-null value alongside this.instance being set to a non-null value.
+      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
       vm.env.scheduleUpdateModifier(instance!);
       this.lastUpdated = valueForTag(tag);
     }