Feature request

PHPStan has a StrContainingTypeSpecifyingExtension but nothing about the returnType.

In the following example, the return type is known
https://phpstan.org/r/31113597-4171-447b-ab83-4eb6ceaa1c66

Some rules could be written

• For constants strings, it could be computed
• For lowercase-string, if the needle is not a lowercase-string it will be false
• For uppercase-string, if the needle is not a uppercase-string it will be false
• For numeric-string, the needle should be compatible with the format [whitespace] [+/-] [digits] [.digits] [e/E [+/-] digits] [whitespace]

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