UserLoginTestBase::assertFailedLogin in core/modules/user/tests/user.test
Make an unsuccessful login attempt.
UserLoginTestCase::testLoginMethods in core/modules/user/tests/user.test
Test that login credentials work (or not) in different login modes.
user_login in core/modules/user/user.module
Form builder; the main user login form.
user_login_block in core/modules/user/user.module
user_login_name_validate in core/modules/user/user.module
First phase validation handler for the login form.
user_login_settings in core/modules/user/user.admin.inc
Login settings form.