Clarify errs.Join nil behavior and update zap example

.github/copilot-instructions.md

@@ -35,5 +35,6 @@
 
 ## API and Compatibility Notes
 - Do not introduce new usage of deprecated `Cause()`; prefer `errors.Is`/`errors.As` compatible flows and `Unwrap`/`Unwraps`.
+- Keep `errs.Join` behavior stable: ignore nil arguments and return nil when all arguments are nil.
 - Treat changes to exported symbols, function signatures, and observable error formatting behavior as potentially breaking.
 - Error string and JSON formatting are validated by tests; when output behavior changes, update README examples and test expectations in the same change.

README.md

@@ -82,6 +82,7 @@ For multiple causes, it returns all causes as a slice.
 - `errs.New("")` returns `nil`
 - `errs.Wrap(nil)` returns `nil`
 - If `WithCause` is given multiple times, the last cause is used
+- `errs.Join(...)` ignores `nil` arguments and returns `nil` if all arguments are `nil`
 
 ### Create new error instance with cause
 

errlist.go

@@ -17,6 +17,8 @@ type Errors struct {
 }
 
 // Join function returns Errors instance.
+//
+// Join ignores nil elements in arguments and returns nil if all elements are nil.
 func Join(errlist ...error) error {
 	if len(errlist) == 0 {
 		return nil

zapobject/example_test.go

@@ -10,7 +10,7 @@ import (
 	"go.uber.org/zap"
 )
 
-func checkFileOpen(path string) error {
+func checkFileOpen(path string) (retErr error) {
 	file, err := os.Open(path)
 	if err != nil {
 		return errs.New(
@@ -19,7 +19,15 @@ func checkFileOpen(path string) error {
 			errs.WithContext("path", path),
 		)
 	}
-	defer file.Close()
+	defer func() {
+		if err := file.Close(); err != nil {
+			closeErr := errs.Wrap(
+				err,
+				errs.WithContext("path", path),
+			)
+			retErr = errs.Join(retErr, closeErr)
+		}
+	}()
 
 	return nil
 }
@@ -41,7 +49,7 @@ func generateMultiError() error {
 
 func Example() {
 	logger := zap.NewExample()
-	defer logger.Sync()
+	defer func() { _ = logger.Sync() }()
 
 	if err := checkFileOpen("not-exist.txt"); err != nil {
 		logger.Error("err", zap.Object("error", zapobject.New(err)))