assert does not constrain to string union to evaluate accessed value type under array key #4747
Description
Bug report
asserting $var to be of value 'x' does not help phpstan to understand that $a['a'.$var] accesses $a['ax'].
So for $a = array{ax: int, ay: string}, phpstan does not know $a['a'.$var] is of type int (thinks int|string instead).