Fix phpstan/phpstan#14413: Narrowing generic union via ::class comparison discards type parameters

[mhert]

Summary

Narrowing a generic union like Cat<string>|Dog<string> via $a::class === Cat::class discarded the generic type parameters — the inferred type became plain Cat instead of Cat<string>. This caused downstream type errors when accessing generic methods on the narrowed variable.

Motivation

When using ::class comparisons to narrow generic union types, PHPStan's TypeSpecifier::resolveNormalizedIdentical() constructed a plain ObjectType without consulting the variable's known type for generic information. This meant that code like:

/** @param Cat<string>|Dog<string> $a */
function foo(Animal $a): void {
    if ($a::class === Cat::class) {
        $a->value(); // was inferred as mixed instead of string
    }
}

lost all generic type information after narrowing.

Changes

• src/Analyser/TypeSpecifier.php — Extracted the duplicated $a::class === 'Foo' / 'Foo' === $a::class handling into a single resolveClassStringComparison() helper. Added generic-aware narrowing that infers template type parameters from the variable's current type via getAncestorWithClassName() + inferTemplateTypes().

Test Cases

• tests/PHPStan/Analyser/nsrt/class-string-generic-narrowing.php — Multiple scenarios:
  • Union match preserves generics (Cat<string>|Dog<string> → Cat<string> in match arm)
  • If/else ::class comparison preserves generics
  • Mirror case (Cat::class === $a::class) preserves generics
  • Match with method call on narrowed generic type
  • Non-matching class comparison yields *NEVER*

Fixes phpstan/phpstan#14413

Co-Authored-By: Claude Code

[ondrejmirtes]

Please open an issue with a reproducing code example first.

[phpstan-bot]

This pull request has been marked as ready for review.

[mhert]

Friendly ping for review when you get a chance 😊