added demo file

example/shirtsize.go

@@ -2,7 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package testdata
+package main
+
+import (
+	"encoding/json"
+	"log"
+	"os"
+)
 
 //go:generate jsonenums -type=ShirtSize
 type ShirtSize byte
@@ -28,3 +34,13 @@ const (
 	Saturday
 	Sunday
 )
+
+func main() {
+	v := struct {
+		Size ShirtSize
+		Day  WeekDay
+	}{M, Friday}
+	if err := json.NewEncoder(os.Stdout).Encode(v); err != nil {
+		log.Fatal(err)
+	}
+}

example/shirtsize_jsonenums.go

@@ -1,6 +1,6 @@
 // generated by jsonenums -type=ShirtSize; DO NOT EDIT
 
-package testdata
+package main
 
 import (
 	"encoding/json"
@@ -63,7 +63,7 @@ func (r *ShirtSize) UnmarshalJSON(data []byte) error {
 		*r = XL
 
 	default:
-		return nil, fmt.Errorf("invalid ShirtSize %q", s)
+		return fmt.Errorf("invalid ShirtSize %q", s)
 	}
 	return nil
 }

example/weekday_jsonenums.go

@@ -1,6 +1,6 @@
 // generated by jsonenums -type=WeekDay; DO NOT EDIT
 
-package testdata
+package main
 
 import (
 	"encoding/json"
@@ -69,7 +69,7 @@ func (r *WeekDay) UnmarshalJSON(data []byte) error {
 		*r = Sunday
 
 	default:
-		return nil, fmt.Errorf("invalid WeekDay %q", s)
+		return fmt.Errorf("invalid WeekDay %q", s)
 	}
 	return nil
 }