Decorate: report annotation build errors instead of ignoring them

CHANGELOG.md

@@ -11,7 +11,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
-- No changes yet.
+
+### Fixed
+- Errors from building an annotated function passed to `fx.Decorate` or `fx.Replace`
+  are now reported instead of being silently ignored together with the decorator.
 
 ## [1.24.0](https://github.com/uber-go/fx/compare/v1.23.0...v1.24.0) - 2025-05-13
 

decorate.go

@@ -227,9 +227,11 @@ func runDecorator(c container, d decorator, opts ...dig.DecorateOption) (err err
 
 	switch decorator := decorator.(type) {
 	case annotated:
-		if dcor, derr := decorator.Build(); derr == nil {
-			err = c.Decorate(dcor, opts...)
+		var dcor any
+		if dcor, err = decorator.Build(); err != nil {
+			return
 		}
+		err = c.Decorate(dcor, opts...)
 	default:
 		err = c.Decorate(decorator, opts...)
 	}

decorate_test.go

@@ -22,6 +22,7 @@ package fx_test
 
 import (
 	"errors"
+	"io"
 	"strings"
 	"testing"
 
@@ -442,6 +443,29 @@ func TestDecorateFailure(t *testing.T) {
 		assert.Contains(t, err.Error(), "major sadness")
 	})
 
+	t.Run("annotation build error on a decorator is reported", func(t *testing.T) {
+		type Logger struct {
+			Name string
+		}
+
+		app := NewForTest(t,
+			fx.Provide(func() *Logger {
+				return &Logger{Name: "root"}
+			}),
+			fx.Decorate(fx.Annotate(
+				func(l *Logger) *Logger { return l },
+				// *Logger does not implement io.Reader, so building
+				// this annotated decorator must fail.
+				fx.As(new(io.Reader)),
+			)),
+			fx.Invoke(func(l *Logger) {}),
+		)
+
+		err := app.Err()
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "does not implement")
+	})
+
 	t.Run("all decorator dependencies must be provided", func(t *testing.T) {
 		type Logger struct {
 			Name string