Fix match exhaustiveness and generic preservation for ::class comparisons

[mhert]

Summary

Two related fixes for ::class comparison type narrowing in match expressions and if-else conditions.

1. match($foo::class) on a @phpstan-sealed hierarchy falsely reported "Match expression does not handle remaining value" even when all allowed subtypes were covered.
2. Narrowing a generic union like Cat<string>|Dog<string> via $a::class === Cat::class discarded generic type parameters — the type became plain Cat instead of Cat<string>.

Closes phpstan/phpstan#12241

Root Cause

Sealed exhaustiveness: GenericClassStringType::tryRemove() only handled final classes — it had no awareness of @phpstan-sealed hierarchies. Removing a class-string constant for a non-final sealed subtype was a no-op, so the type was never fully exhausted to never.

Generic preservation: The $a::class === 'Foo' handler in TypeSpecifier::resolveNormalizedIdentical() constructed a plain ObjectType without consulting the current variable type for generic information. The generic parameters from the original union were lost.

Fix

Sealed exhaustiveness: Extended GenericClassStringType::tryRemove() to consult ClassReflection::getAllowedSubTypes(). Each match arm progressively subtracts one allowed subtype via ObjectType subtraction. When all subtypes are removed, the type becomes never, making the match exhaustive. Special-cases concrete (non-abstract) sealed parents to avoid self-referential subtraction.

Generic preservation: Added TypeSpecifier::narrowTypePreservingGenerics() which preserves generics through two strategies:

• For union types: TypeCombinator::intersect() distributes over members and picks the matching generic member
• For single generic parents: infers child template parameters through the class hierarchy via inferTemplateTypes()

Test Plan

• MatchExpressionRuleTest::testSealedClassStringMatch — exhaustive matches produce no error, partial matches report remaining value
• nsrt/sealed-class-string-match.php — progressive class-string narrowing, concrete parents, sealed interfaces, falsy branch (!==), match expressions
• nsrt/class-string-match-preserves-generics.php — generic preservation for unions, single parents, final classes, mirror syntax, match arms with method calls
• All existing tests pass — no regressions

Co-Authored-By: Claude Code

[phpstan-bot]

You've opened the pull request against the latest branch 2.2.x. PHPStan 2.2 is not going to be released for months. If your code is relevant on 2.1.x and you want it to be released sooner, please rebase your pull request and change its target to 2.1.x.

[phpstan-bot]

This pull request has been marked as ready for review.