fx.Annotate: make variadic params optional by default

annotated.go

@@ -360,7 +360,7 @@ func (ann *annotated) parameters() (
 
 	// No parameter annotations. Return the original types
 	// and an identity function.
-	if len(ann.ParamTags) == 0 {
+	if len(ann.ParamTags) == 0 && !ft.IsVariadic() {
 		return types, func(args []reflect.Value) []reflect.Value {
 			return args
 		}
@@ -383,6 +383,11 @@ func (ann *annotated) parameters() (
 
 		if i < len(ann.ParamTags) {
 			field.Tag = reflect.StructTag(ann.ParamTags[i])
+		} else if i == ft.NumIn()-1 && ft.IsVariadic() {
+			// If a variadic argument is unannotated, mark it optional,
+			// so that just wrapping a function in fx.Annotate does not
+			// suddenly introduce a required []arg dependency.
+			field.Tag = reflect.StructTag(`optional:"true"`)
 		}
 
 		inFields = append(inFields, field)

annotated_test.go

@@ -522,6 +522,10 @@ func TestAnnotate(t *testing.T) {
 	newSliceA := func(sa ...*a) *sliceA {
 		return &sliceA{sa}
 	}
+	newSliceAWithB := func(b *b, sa ...*a) *sliceA {
+		total := append(sa, b.a)
+		return &sliceA{total}
+	}
 
 	t.Run("Provide with optional", func(t *testing.T) {
 		t.Parallel()
@@ -599,6 +603,72 @@ func TestAnnotate(t *testing.T) {
 		assert.Len(t, got.sa, 2)
 	})
 
+	t.Run("Provide variadic function with no optional params", func(t *testing.T) {
+		t.Parallel()
+
+		var got struct {
+			fx.In
+
+			Result *sliceA `name:"as"`
+		}
+		app := fxtest.New(t,
+			fx.Supply([]*a{{}, {}, {}}),
+			fx.Provide(
+				fx.Annotate(newSliceA,
+					fx.ResultTags(`name:"as"`),
+				),
+			),
+			fx.Populate(&got),
+		)
+		defer app.RequireStart().RequireStop()
+		require.NoError(t, app.Err())
+		assert.Len(t, got.Result.sa, 3)
+	})
+
+	t.Run("Provide variadic function named with no given params", func(t *testing.T) {
+		t.Parallel()
+
+		var got *sliceA
+		app := NewForTest(t,
+			fx.Provide(
+				fx.Annotate(newSliceA, fx.ParamTags(`name:"a"`)),
+			),
+			fx.Populate(&got),
+		)
+		err := app.Err()
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), `missing dependencies`)
+		assert.Contains(t, err.Error(), `missing type: []*fx_test.a[name="a"]`)
+	})
+
+	t.Run("Invoke variadic function with multiple params", func(t *testing.T) {
+		t.Parallel()
+
+		app := fxtest.New(t,
+			fx.Supply(
+				fx.Annotate(newB(newA()), fx.ResultTags(`name:"b"`)),
+			),
+			fx.Invoke(fx.Annotate(newSliceAWithB, fx.ParamTags(`name:"b"`))),
+		)
+
+		defer app.RequireStart().RequireStop()
+		require.NoError(t, app.Err())
+	})
+
+	t.Run("Invoke non-optional variadic function with a missing dependency", func(t *testing.T) {
+		t.Parallel()
+
+		app := NewForTest(t,
+			fx.Invoke(
+				fx.Annotate(newSliceA, fx.ParamTags(`optional:"false"`)),
+			),
+		)
+		err := app.Err()
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), `missing dependencies`)
+		assert.Contains(t, err.Error(), `missing type: []*fx_test.a`)
+	})
+
 	t.Run("Invoke with variadic function", func(t *testing.T) {
 		t.Parallel()