Validate reassign user in user delete command

features/user.feature

@@ -136,6 +136,49 @@ Feature: Manage WordPress users
       3
       """
 
+  Scenario: Delete user with invalid reassign
+    Given a WP install
+    And a session_no file:
+      """
+      n
+      """
+    And a session_yes file:
+      """
+      y
+      """
+
+    When I run `wp user create bobjones bob@example.com --role=author --porcelain`
+    And save STDOUT as {BOB_ID}
+
+    When I run `wp post list --format=count`
+    And save STDOUT as {TOTAL_POSTS}
+
+    When I run `wp post generate --count=3 --format=ids --post_author=bobjones`
+    And I run `wp post list --author={BOB_ID} --format=count`
+    Then STDOUT should be:
+      """
+      3
+      """
+
+    When I run `wp user delete bobjones < session_no`
+    Then STDOUT should contain:
+      """
+      --reassign parameter not passed. All associated posts will be deleted. Proceed? [y/n]
+      """
+
+    When I run `wp user delete bobjones --reassign=99999 < session_no`
+    Then STDOUT should contain:
+      """
+      --reassign parameter is invalid. All associated posts will be deleted. Proceed? [y/n]
+      """
+
+    When I run `wp user delete bobjones < session_yes`
+    And I run `wp post list --format=count`
+    Then STDOUT should be:
+      """
+      {TOTAL_POSTS}
+      """
+
   Scenario: Deleting user from the whole network
     Given a WP multisite install
 

src/User_Command.php

@@ -280,8 +280,12 @@ public function delete( $args, $assoc_args ) {
 			WP_CLI::error( 'Reassigning content to a different user is not supported on multisite.' );
 		}
 
+		$is_reassign_valid = ( $reassign && false === get_userdata( $reassign ) ) ? false : true;
+
 		if ( ! $reassign ) {
 			WP_CLI::confirm( '--reassign parameter not passed. All associated posts will be deleted. Proceed?', $assoc_args );
+		} elseif ( ! $is_reassign_valid ) {
+			WP_CLI::confirm( '--reassign parameter is invalid. All associated posts will be deleted. Proceed?', $assoc_args );
 		}
 
 		$users = $this->fetcher->get_many( $args );