Recognize covariance when returning generic union types

[spawnia]

Bug report

See #5841

Code snippet that reproduces the problem

https://phpstan.org/r/f64c26bf-9543-4f57-9024-a415d78b2cd2

Expected output

PHPStan recognizes the narrowing of the type:

Function value() should return T of int|string but returns T of string.

However, it does not seem to recognize that T of string is covariant to T of int|string.

Separating the generics and doing an ad-hoc union seems to work: https://phpstan.org/r/673e1385-a14a-47b5-9102-a7d3d3e3ce96

Did PHPStan help you today? Did it make you happy in any way?

PHPStan level 4 found a lot of redundant type checks in https://github.com/webonyx/graphql-php. A series of pull requests driven by the analysis lead to a 10-20% performance boost.

[ondrejmirtes]

/cc @arnaud-lb This bug also looks like something up your alley :)

[github-actions]

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