This is very very valuable, but it needs two things:

1. The new errors can be reported only with bleeding edge.
2. The new code has to employ RuleLevelHelper. The idea is that "always wrong" like int is always reported, "sometimes wrong" like int|string is reported on level 7+, nullable like string|null is reported on level 8+ and mixeds are reported on level 9+. Just use RuleLevelHelper like it's used in all the rules and it will be taken care of automatically.

Also please do this in two separate PRs:

1. Let the rules handle Expr as $node->name to continue the error-reporting. So that:

$a = 'Foo';
echo Foo::{$a}

Is checked against a non-existent constant. This doesn't need bleeding edge.

2. Introduce new reported errors that check $a in Foo::{$a} to be a string. This needs bleeding edge and RuleLevelHelper.

Thank you.