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README.md

isSourceFileDefaultLibrary returns false after program structure reuse

Minimal reproduction showing that program.isSourceFileDefaultLibrary() returns false after full program structure reuse via oldProgram in TypeScript 6.

What causes this

When tryReuseStructureFromOldProgram returns StructureIsReused.Completely, the new program never populates its libFiles Set and doesn't copy it from the old program. This means isSourceFileDefaultLibrary — which checks membership in that set — always returns false for the reused program.

Caused by microsoft/TypeScript#62435

Running the reproduction

pnpm install
pnpm test

The test suite runs checkIsSourceFileDefaultLibraryAfterReuse against three TypeScript versions:

  • typescript-5: TypeScript 5.9.3
  • typescript-6: TypeScript 6.0.0-beta
  • typescript-6-patched: TypeScript 6.0.1-rc with a quick-and-dirty patch to validate the bug.

I'm using rc for the patch version only to be able to have both patched and unpatched versions in the same project, it doesn't have anything to do with the bug itself.

AI Disclaimer

I used AI to generate the reproduction to save some time. The bug and the solution were discovered manually by me when working on ts-eslint upgrade to TypeScript 6 (typescript-eslint/typescript-eslint#12124).