The first parameter of user_get_user_details must be a user record (from mdl_user) as indicated in the function documentation.

It happens that the message API is calling this function via show_online_status passing as parameter partial user objects that are not user records from the DB

I discovered this while working in a fix for MDL-63395

Some errors, in this case the user object is missing the deleted field from the user table:

1) core_message_api_testcase::test_search_users_in_course
Undefined property: stdClass::$deleted

/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:1153
/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:272
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:338
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:317
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/api.php:185
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/tests/api_test.php:164
/Users/juanleyvadelgado/moodles/stable_master/moodle/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
vendor/bin/phpunit "core_message_api_testcase" message/tests/api_test.php

2) core_message_api_testcase::test_search_users
Undefined property: stdClass::$deleted

/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:1153
/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:272
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:338
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:317
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/api.php:229
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/tests/api_test.php:262
/Users/juanleyvadelgado/moodles/stable_master/moodle/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
vendor/bin/phpunit "core_message_api_testcase" message/tests/api_test.php

3) core_message_api_testcase::test_search_messages
Undefined property: stdClass::$deleted

/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:1153
/Users/juanleyvadelgado/moodles/stable_master/moodle/user/lib.php:272
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:338
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/helper.php:317
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/classes/api.php:144
/Users/juanleyvadelgado/moodles/stable_master/moodle/message/tests/api_test.php:303
/Users/juanleyvadelgado/moodles/stable_master/moodle/lib/phpunit/classes/advanced_testcase.php:80