Worked on the apple/pkl repository to enhance compiler robustness by implementing compile-time validation that rejects abstract members in non-abstract classes and modules. Leveraging Java and compiler design expertise, introduced AST-level enforcement within the AstBuilder to align language semantics with established standards and reduce runtime errors. Developed user-facing error messages that precisely indicate violations at the 'abstract' keyword, improving developer feedback. Expanded regression and module-level tests to ensure comprehensive coverage of abstract member scenarios, and updated documentation fixtures accordingly. Collaborated on code reviews and verified changes through full builds, achieving a build-health milestone and documenting the rationale for future enforcement.