TypeScript 6 API Bug - `isSourceFileDefaultLibrary` false negative on program structure reuse

[StyleShit]

🔎 Search Terms

isSourceFileDefaultLibrary

🕗 Version & Regression Information

• This changed between versions 5.9.3 and 6.0.0-beta
• This changed in commit or PR - Remove no-default-lib #62435

⏯ Playground Link

No response

💻 Code

https://github.com/StyleShit/repros/tree/main/ts-6-isSourceFileDefaultLibrary

🙁 Actual behavior

isSourceFileDefaultLibrary returns false for source files in the default library

🙂 Expected behavior

isSourceFileDefaultLibrary will return true for source files in the default library, as in previous versions

Additional information about the issue

I'm willing to send a PR to fix this

[RyanCavanaugh]

Huh, this is very weird. It seems like it'd have some obvious effects under watch mode.

[jakebailey]

@StyleShit I think #63239 (comment) should fix it.

[StyleShit]

@jakebailey works for me in the ts-eslint PR 👍

[paigefletcher1282-code]

It did have problems under the watch mode

…

On Thu, Mar 12, 2026, 3:56 PM Jake Bailey ***@***.***> wrote: Closed #63228 <#63228> as completed via #63239 <#63239>. — Reply to this email directly, view it on GitHub <#63228 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/B7BB4JT3UPNKPWS3J6UX5FT4QLMZZAVCNFSM6AAAAACWND6IXWVHI2DSMVQWIX3LMV45UABCJFZXG5LFIV3GK3TUJZXXI2LGNFRWC5DJN5XDWMRTGUYTGOBXGI2DINA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>