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