Fix decorator not being applied to transient dependencies

decorate_test.go

@@ -313,6 +313,39 @@ func TestDecorateSuccess(t *testing.T) {
 		)
 		defer app.RequireStart().RequireStop()
 	})
+
+	t.Run("decoration must execute when required by a member of group", func(t *testing.T) {
+		type Drinks interface {
+		}
+		type Coffee struct {
+			Type  string
+			Name  string
+			Price int
+		}
+		type PriceService struct {
+			DefaultPrice int
+		}
+		app := fxtest.New(t,
+			fx.Provide(func() *PriceService {
+				return &PriceService{DefaultPrice: 3}
+			}),
+			fx.Decorate(func(service *PriceService) *PriceService {
+				service.DefaultPrice = 10
+				return service
+			}),
+			fx.Provide(fx.Annotate(func(service *PriceService) Drinks {
+				assert.Equal(t, 10, service.DefaultPrice)
+				return &Coffee{Type: "coffee", Name: "Americano", Price: service.DefaultPrice}
+			}, fx.ResultTags(`group:"drinks"`))),
+			fx.Provide(fx.Annotated{Group: "drinks", Target: func() Drinks {
+				return &Coffee{Type: "coffee", Name: "Cold Brew", Price: 4}
+			}}),
+			fx.Invoke(fx.Annotate(func(drinks []Drinks) {
+				assert.Len(t, drinks, 2)
+			}, fx.ParamTags(`group:"drinks"`))),
+		)
+		defer app.RequireStart().RequireStop()
+	})
 }
 
 func TestDecorateFailure(t *testing.T) {

module.go

@@ -81,23 +81,32 @@ func (o moduleOption) apply(mod *module) {
 type module struct {
 	parent     *module
 	name       string
-	scope      *dig.Scope
+	scope      scope
 	provides   []provide
 	invokes    []invoke
 	decorators []decorator
 	modules    []*module
 	app        *App
 }
 
+// scope is a private wrapper interface for dig.Container and dig.Scope.
+// We can consider moving this into Fx using type constraints after Go 1.20
+// is released and 1.17 is deprecated.
+type scope interface {
+	Decorate(f interface{}, opts ...dig.DecorateOption) error
+	Invoke(f interface{}, opts ...dig.InvokeOption) error
+	Provide(f interface{}, opts ...dig.ProvideOption) error
+	Scope(name string, opts ...dig.ScopeOption) *dig.Scope
+	String() string
+}
+
 // builds the Scopes using the App's Container. Note that this happens
 // after applyModules' are called because the App's Container needs to
 // be built for any Scopes to be initialized, and applys' should be called
 // before the Container can get initialized.
 func (m *module) build(app *App, root *dig.Container) {
 	if m.parent == nil {
-		m.scope = root.Scope(m.name)
-		// TODO: Once fx.Decorate is in-place,
-		// use the root container instead of subscope.
+		m.scope = root
 	} else {
 		parentScope := m.parent.scope
 		m.scope = parentScope.Scope(m.name)