diff --git a/core/modules/user/lib/Drupal/user/Tests/UserEditTest.php b/core/modules/user/lib/Drupal/user/Tests/UserEditTest.php
index 6bf60bc..0a5fcd5 100644
--- a/core/modules/user/lib/Drupal/user/Tests/UserEditTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/UserEditTest.php
@@ -99,4 +99,30 @@ class UserEditTest extends WebTestBase {
     $this->drupalPost("user/$user1->uid/edit", array('mail' => ''), t('Save'));
     $this->assertRaw(t("The changes have been saved."));
   }
+
+  /**
+   * Check that existing users whose username matches another user's email
+   * address or vice versa are not forced to update their username or email
+   * address.
+   */
+  function testUserEditWithConflicts() {
+    $user_with_email = $this->drupalCreateUser();
+    $user_with_name = $this->drupalCreateUser();
+
+    // Change the second user's username to the same value as the first user's
+    // email address.
+    $user_with_name->name = $user_with_email->mail;
+    $user_with_name->save();
+
+    // Test that the first user can save their account with no errors.
+    $this->drupalLogin($user_with_email);
+    $this->drupalPost("user/$user_with_email->uid/edit", array(), t('Save'));
+    $this->assertText(t("The changes have been saved."), "The user does not need to change their username if it matches another user's email address.");
+    $this->drupalLogout();
+
+    // Test that the second user can save their account with no errors.
+    $this->drupalLogin($user_with_name);
+    $this->drupalPost("user/$user_with_name->uid/edit", array(), t('Save'));
+    $this->assertText(t("The changes have been saved."), "The user does not need to change their email address if it matches another user's username.");
+  }
 }
diff --git a/core/modules/user/lib/Drupal/user/Tests/UserPasswordResetTest.php b/core/modules/user/lib/Drupal/user/Tests/UserPasswordResetTest.php
index 00c947f..85a95ef 100644
--- a/core/modules/user/lib/Drupal/user/Tests/UserPasswordResetTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/UserPasswordResetTest.php
@@ -33,26 +33,14 @@ class UserPasswordResetTest extends WebTestBase {
     // Don't require admin approval for new accounts.
     variable_set('user_register', USER_REGISTER_VISITORS);
 
-    // Create a user to be duplicated.
+    // Create two users.
     $user_with_email = $this->drupalCreateUser();
+    $user_with_name = $this->drupalCreateUser();
 
-    // Create a new user with the email from the above as the user's name.
-    $edit = array();
-    $edit['name'] = $user_with_email->mail;
-    $edit['mail'] = $this->randomName() . "@example.com";
-    $edit['pass[pass1]'] = $new_pass = $this->randomName();
-    $edit['pass[pass2]'] = $new_pass;
-    $this->drupalPost('user/register', $edit, t('Create new account'));
-    // For backward compatibility, registration with a username that conflicts
-    // with an email address of another account should be allowed.
-    $this->assertText(t('Registration successful. You are now logged in.'), t('Users are logged in after registering.'));
-    // The above logs in the user.  Log out to test password reset.
-    $this->drupalLogout();
-
-    // Get the duplicate user.
-    $users = user_load_multiple(array(), array('name' => $edit['name'], 'status' => '1'));
-    $user_with_name = reset($users);
-
+    // Change the second user's username to the same value as the first user's
+    // email address.
+    $user_with_name->name = $user_with_email->mail;
+    $user_with_name->save();
 
     // Try and reset based on the duplicated email.
     $edit = array();
@@ -76,7 +64,6 @@ class UserPasswordResetTest extends WebTestBase {
 
     // If the user is logged in, they should not have to choose an account to
     // reset their password.
-    $user_with_name->pass_raw = $new_pass;
     $this->drupalLogin($user_with_name);
     $this->drupalGet('user/password');
     // There should not be a form element for name.
diff --git a/core/modules/user/lib/Drupal/user/Tests/UserRegistrationTest.php b/core/modules/user/lib/Drupal/user/Tests/UserRegistrationTest.php
index 56ad9a0..b51d941 100644
--- a/core/modules/user/lib/Drupal/user/Tests/UserRegistrationTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/UserRegistrationTest.php
@@ -137,6 +137,43 @@ class UserRegistrationTest extends WebTestBase {
     $this->assertText(t('The e-mail address @email is already registered.', array('@email' => $duplicate_user->mail)), t('Supplying a duplicate email address with added whitespace displays an error message'));
   }
 
+  /**
+   * Ensure that a new account cannot be created when the username matches an
+   * existing user's email address, or when the email address matches an
+   * existing user's username.
+   */
+  function testRegistrationConflicts() {
+    // Don't require e-mail verification.
+    variable_set('user_email_verification', FALSE);
+
+    // Allow registration by site visitors without administrator approval.
+    variable_set('user_register', USER_REGISTER_VISITORS);
+
+    // Set up a user to check for duplicates.
+    $duplicate_user = $this->drupalCreateUser();
+
+    $edit = array();
+    $edit['name'] = $duplicate_user->mail;
+    $edit['mail'] = $this->randomName() . '@example.com';
+
+    // Attempt to create a new account using a username that matches an
+    // existing email.
+    $this->drupalPost('user/register', $edit, t('Create new account'));
+    $this->assertText(t('The name @name is already taken.', array('@name' => $edit['name'])), "A user cannot be created when their username matches an existing user's email address.");
+
+    // Change the username to an email address.
+    $duplicate_user->name = $name = $this->randomName() . '@example.com';
+    $duplicate_user->save();
+
+    $edit['name'] = $this->randomName();
+    $edit['mail'] = $name;
+
+    // Attempt to create a new account using an email that matches an existing
+    // username.
+    $this->drupalPost('user/register', $edit, t('Create new account'));
+    $this->assertText(t('The e-mail address @email is already registered.', array('@email' => $edit['mail'])), "A user cannot be created when their email address matches an existing username.");
+  }
+
   function testRegistrationDefaultValues() {
     // Allow registration by site visitors without administrator approval.
     variable_set('user_register', USER_REGISTER_VISITORS);
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 519500e..e34f1a2 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -966,6 +966,9 @@ function user_validate_current_pass(&$form, &$form_state) {
  */
 function user_account_form_validate($form, &$form_state) {
   $account = $form['#user'];
+  $name_taken = FALSE;
+  $mail_taken = FALSE;
+
   // Validate new or changing username.
   if (isset($form_state['values']['name'])) {
     if ($error = user_validate_name($form_state['values']['name'])) {
@@ -982,7 +985,18 @@ function user_account_form_validate($form, &$form_state) {
         ->execute()
         ->fetchField();
 
-      if ($name_taken) {
+      // For new registrations, make sure the username does not conflict with an
+      // existing user's email address.
+      if (empty($account->uid)) {
+        $mail_taken = (bool) db_select('users')
+          ->fields('users', array('uid'))
+          ->condition('mail', db_like($form_state['values']['name']), 'LIKE')
+          ->range(0, 1)
+          ->execute()
+          ->fetchField();
+      }
+
+      if ($name_taken || $mail_taken) {
         form_set_error('name', t('The name %name is already taken.', array('%name' => $form_state['values']['name'])));
       }
     }
@@ -999,7 +1013,18 @@ function user_account_form_validate($form, &$form_state) {
       ->execute()
       ->fetchField();
 
-    if ($mail_taken) {
+    // For new registrations, make sure the email address does not conflict with
+    // an existing user's username.
+    if (empty($account->uid)) {
+      $name_taken = (bool) db_select('users')
+        ->fields('users', array('uid'))
+        ->condition('name', db_like($mail), 'LIKE')
+        ->range(0, 1)
+        ->execute()
+        ->fetchField();
+    }
+
+    if ($mail_taken || $name_taken) {
       // Format error message dependent on whether the user is logged in or not.
       if ($GLOBALS['user']->uid) {
         form_set_error('mail', t('The e-mail address %email is already taken.', array('%email' => $mail)));