Bring back the rest-user-exists-controller file

changelog/fix-plugin-upgrade

@@ -0,0 +1,5 @@
+Significance: patch
+Type: fix
+Comment: Temporary fix to hide an error notice that's printed during the plugin upgrade
+
+

includes/admin/class-wc-rest-user-exists-controller.php

@@ -0,0 +1,78 @@
+<?php
+/**
+ * Class WC_REST_User_Exists_Controller
+ *
+ * @package WooCommerce\Payments\Admin
+ */
+
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * REST controller to check if a user exists.
+ */
+class WC_REST_User_Exists_Controller extends WP_REST_Controller {
+
+	/**
+	 * Endpoint namespace.
+	 *
+	 * @var string
+	 */
+	protected $namespace = 'wc/v3';
+
+	/**
+	 * Endpoint path.
+	 *
+	 * @var string
+	 */
+	protected $rest_base = 'users/exists';
+
+	/**
+	 * Configure REST API routes.
+	 */
+	public function register_routes() {
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base,
+			[
+				'methods'             => WP_REST_Server::CREATABLE,
+				'callback'            => [ $this, 'user_exists' ],
+				'permission_callback' => '__return_true',
+				'args'                => [
+					'email' => [
+						'required'    => true,
+						'description' => __( 'Email address.', 'woocommerce-payments' ),
+						'type'        => 'string',
+						'format'      => 'email',
+					],
+				],
+			]
+		);
+	}
+
+	/**
+	 * Retrieve if a user exists by email address.
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 *
+	 * @return WP_REST_Response
+	 */
+	public function user_exists( WP_REST_Request $request ): WP_REST_Response {
+		$email        = $request->get_param( 'email' );
+		$email_exists = ! empty( email_exists( $email ) );
+		$message      = null;
+
+		if ( $email_exists ) {
+			// Use this function to show the core error message.
+			$error   = wc_create_new_customer( $email );
+			$message = $error->get_error_message();
+		}
+
+		return new WP_REST_Response(
+			[
+				'user-exists' => $email_exists,
+				'message'     => $message,
+			]
+		);
+	}
+}
+