Added regression test

[staabm]

for https://github.com/phpstan/phpstan/issues/1016#issuecomment-727048118

[rvanvelzen]

This is really testing the exact same situation as the test for the original case, which is why I didn't include it.

[staabm]

I agree its the very same code executed, but from a user point of view the example contains different types/concepts.

IMO it deserves a separate test

[ondrejmirtes]

I have no strong opinion one way or the other, so I'll merge this. Thanks!