Add PHP 8 Union Types Support for function Return Type

[karyna-t]

In PHP 8, new Union types feature was introduced.
(see details https://wiki.php.net/rfc/union_types_v2)

e.g. it's possible to create a function with a signature like
function baz(): Foo|Bar|int { ...
were return type is a union.

Right now Magento will not work properly with such a feature. There are several places where we're using Parameter's Class (e.g. for code generation):

• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Async/Code/Generator/ProxyDeferredGenerator.php#L247
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/Interception/Code/Generator/Interceptor.php#L213
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/MessageQueue/Code/Generator/RemoteServiceGenerator.php#L158
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/ObjectManager/Code/Generator/Proxy.php#L288
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/ObjectManager/Code/Generator/Repository.php#L252
• https://github.com/magento/magento2/blob/platform-health/lib/internal/Magento/Framework/ObjectManager/Code/Generator/Repository.php#L726
• \Magento\Framework\Reflection\Test\Unit\TypeProcessorTest
  (better to re-check other possible places)

Such places should be adjusted to work properly with union types.
The main reason for this adjustments it's because 3rd-party developers may develop some new extensions and declare them to support only php >= 8.0.
in such modules they may use this PHP8-feature.

Expected result (*)

Magento should not break when union types are used in the function return type. It should properly generate code and so on.

Actual result (*)

Right now Magento can't handle union types introduced in php 8.

Related issues:

#34263
#34265

[m2-assistant]

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[m2-assistant]

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[engcom-November]

Hi @karyna-tsymbal-atwix ,
We have gone through the main description and found that this is not related to any bug of Magento but it can be a good feature to have in Magento. Hence marking this ticket as a feature request to process further.
Thank you.