[MM-49180] Error message not clear when creating group with mention name matching username (#22022)

* return the correct error message ID

* add test

Co-authored-by: Mattermost Build <build@mattermost.com>

app/group.go

@@ -6,7 +6,6 @@ package app
 import (
 	"encoding/json"
 	"errors"
-	"fmt"
 	"net/http"
 
 	"github.com/mattermost/mattermost-server/v6/model"
@@ -117,7 +116,7 @@ func (a *App) isUniqueToUsernames(val string) *model.AppError {
 		return model.NewAppError("isUniqueToUsernames", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 	if user != nil {
-		return model.NewAppError("isUniqueToUsernames", model.NoTranslation, nil, fmt.Sprintf("user name %s exists", val), http.StatusBadRequest)
+		return model.NewAppError("isUniqueToUsernames", "app.group.username_conflict", map[string]interface{}{"Username": val}, "", http.StatusBadRequest)
 	}
 	return nil
 }

app/group_test.go

@@ -84,16 +84,19 @@ func TestCreateGroup(t *testing.T) {
 	require.NotNil(t, err)
 	require.Nil(t, g)
 
-	user := th.CreateUser()
-	usernameGroup := &model.Group{
-		DisplayName: "dn_" + model.NewId(),
-		Name:        &user.Username,
-		Source:      model.GroupSourceLdap,
-		RemoteId:    model.NewString(model.NewId()),
-	}
-	g, err = th.App.CreateGroup(usernameGroup)
-	require.NotNil(t, err)
-	require.Nil(t, g)
+	t.Run("should check if the group mention is in use as a username", func(t *testing.T) {
+		user := th.CreateUser()
+		usernameGroup := &model.Group{
+			DisplayName: "dn_" + model.NewId(),
+			Name:        &user.Username,
+			Source:      model.GroupSourceLdap,
+			RemoteId:    model.NewString(model.NewId()),
+		}
+		g, err = th.App.CreateGroup(usernameGroup)
+		require.NotNil(t, err)
+		require.Equal(t, "app.group.username_conflict", err.Id)
+		require.Nil(t, g)
+	})
 }
 
 func TestUpdateGroup(t *testing.T) {

i18n/en.json

@@ -5163,6 +5163,10 @@
     "id": "app.group.uniqueness_error",
     "translation": "group member already exists"
   },
+  {
+    "id": "app.group.username_conflict",
+    "translation": "user with username \"{{.Username}}\" already exists."
+  },
   {
     "id": "app.import.attachment.bad_file.error",
     "translation": "Error reading the file at: \"{{.FilePath}}\""