Hi Nikita, Thank you for your work on this, I look forward to being able to use union types!
The one thing that bothers me a bit is introducing yet another inconsistency by supporting false and not true as a type. Sure, I do get the need for false as a valid type for historical reasons, but at the same time I thought it would be interesting to see if true was being used as well in the wild, before closing the door on it. Therefore I reviewed all Composer packages with > 1 million downloads (1211 packages total), and here are the results: *31 packages <https://gist.github.com/BenMorel/56dfff576b1d8a719cd71b14ef614d2e#file-packages-txt> use at least one @var, @param or @return containing true as a type.* - there are many illegitimate use cases, such as @return true <https://github.com/drupal/core/blob/c447ebb6a89149ef86e0af06bc00fec63027ec0d/modules/views/src/ViewExecutable.php#L719> , @return bool|true <https://github.com/php-http/message/blob/144d13ba024d0c597830636907144bc7c23f50dd/src/Authentication/AutoBasicAuth.php#L23>, @return true|false <https://github.com/zendframework/zend-form/blob/ff4c9ec32d141e4ac622c2aa40a8c9eb77a40725/src/Annotation/AnnotationBuilder.php#L412>, or even @return bool|true|false <https://github.com/slevomat/coding-standard/blob/964a3ff08d7ee924510c3b705f826165adaa97fe/tests/Sniffs/Namespaces/data/fullyQualifiedClassNameInAnnotationNoErrors.php#L41> - there are, however, quite a few potentially legitimate use cases. Among others: - @return true|WP_Error <https://github.com/johnpbloch/wordpress-core/blob/4a96ef28019f3f529465eac71c718afea2c88e8b/wp-includes/rest-api/class-wp-rest-request.php#L641> (*johnpbloch/wordpress-core*) : "True if the JSON data was passed or no JSON data was provided, WP_Error if invalid JSON was passed." - @return true|string <https://github.com/johnpbloch/wordpress-core/blob/bd27b2bc49483b796fdb228f9b0614f9880823ee/wp-includes/ms-load.php#L72> (*johnpbloch/wordpress-core*) : "Returns true on success, or drop-in file to include." - @return string|true <https://github.com/PHP-DI/PHP-DI/blob/b891248f04c594dae4c07650bc0815d0dc81bacb/src/Compiler/Compiler.php#L317> (*php/di*) : True if compilable, string explaining why if not - @return string|true <https://github.com/zendframework/zend-authentication/blob/86d6553e1cfe5ed2bde53cbea9fd162c6875a9c8/src/Adapter/Ldap.php#L378> (*zendframework/zend-authentication*) : True if successfull, error message if not - @param int|true <https://github.com/yiisoft/yii2/blob/fdbe45af3a4f923a97b718d8694981e1e195403d/framework/db/Query.php#L1266> (*yiisoft/yii2*) : "Use 0 to indicate that..., Use a negative number to indicate that..., Use boolean `true` to indicate that ..." - @param callable|resource|string|true|null <https://github.com/symfony/symfony/blob/242f24427d34bd47ef0e3a027a5a4e979d713c6f/src/Symfony/Component/VarDumper/Dumper/AbstractDumper.php#L117> (*symfony/symfony*) : "A line dumper callable, an opened stream, an output path or true to return the dump" Full list here <https://gist.github.com/BenMorel/56dfff576b1d8a719cd71b14ef614d2e#file-types-txt> . So although true as a type does not have the same historical background as false, it does seem that it's being used by enough packages to be worth considering; what do you think? — Benjamin
