an inactive user shouldn't be able to be a collaborator

options/locale/locale_en-US.ini

@@ -1014,6 +1014,7 @@ settings.transfer_succeed = The repository has been transferred.
 settings.confirm_delete = Delete Repository
 settings.add_collaborator = Add Collaborator
 settings.add_collaborator_success = The collaborator has been added.
+settings.add_collaborator_inactive_user = Cannot add an inactive user as a collaborator.
 settings.delete_collaborator = Remove
 settings.collaborator_deletion = Remove Collaborator
 settings.collaborator_deletion_desc = Removing a collaborator will revoke their access to this repository. Continue?

routers/repo/setting.go

@@ -381,6 +381,12 @@ func CollaborationPost(ctx *context.Context) {
 		return
 	}
 
+	if !u.IsActive {
+		ctx.Flash.Error("cannot add an inactive user as a collaborator")
+		ctx.Redirect(setting.AppSubURL + ctx.Req.URL.Path)
+		return
+	}
+
 	// Organization is not allowed to be added as a collaborator.
 	if u.IsOrganization() {
 		ctx.Flash.Error(ctx.Tr("repo.settings.org_not_allowed_to_be_collaborator"))