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
 		}
@@ -381,9 +381,15 @@ func (ann *annotated) parameters() (
 			Type: t,
 		}
 
+		var paramTag string
 		if i < len(ann.ParamTags) {
-			field.Tag = reflect.StructTag(ann.ParamTags[i])
+			paramTag = ann.ParamTags[i]
 		}
+		if i == ft.NumIn()-1 && ft.IsVariadic() && !strings.Contains(paramTag, "optional:") &&
+			!strings.Contains(paramTag, "group:") {
+			paramTag = paramTag + " optional:\"true\""
+		}
+		field.Tag = reflect.StructTag(paramTag)
 
 		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 +603,50 @@ func TestAnnotate(t *testing.T) {
 		assert.Len(t, got.sa, 2)
 	})
 
+	t.Run("Provide variadic function named with no given params", func(t *testing.T) {
+		t.Parallel()
+
+		var got *sliceA
+		app := fxtest.New(t,
+			fx.Provide(
+				fx.Annotate(newSliceA, fx.ParamTags(`name:"a"`)),
+			),
+			fx.Populate(&got),
+		)
+		defer app.RequireStart().RequireStop()
+		require.NoError(t, app.Err())
+
+		assert.Len(t, got.sa, 0)
+	})
+
+	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.Invoke(fx.Annotate(newSliceAWithB)),
+		)
+
+		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()