Mathias contributed to the phpstan/phpstan-src repository by addressing a critical bug in PHPStan’s type system, specifically improving exhaustiveness analysis for sealed class-string matches. He enhanced the handling of match expressions involving ::class comparisons, ensuring all allowed subtypes are correctly recognized and reducing false positives in exhaustive checks. The solution involved updating GenericClassStringType::tryRemove and TypeCombinator::remove to properly subtract sealed hierarchies using ObjectType::changeSubtractedType, aligning with the sealed type model. This work demonstrated depth in PHP, static analysis, and type-system engineering, resulting in more reliable static analysis and safer refactoring for codebases using sealed class-strings.