isProgramUpToDate reports false because it expects HostCache to include typescript/lib/*.d.ts files

[cspotcode]

TypeScript Version: 3.8.0-dev.20200211

Search Terms:

isProgramUpToDate
getScriptFileNames

Code

Full reproduction:
https://github.com/TypeStrong/ts-node-repros/blob/tsIsProgramUpToDate/example.ts
Output:
https://github.com/TypeStrong/ts-node-repros/blob/tsIsProgramUpToDate/output.txt

Briefly, if I create a languageService and repeatedly ask it for the Program, it should return the same program instance each time, provided nothing has changed.

//... create LanguageService using a LanguageServiceHost that does not implement getProjectVersion
assert(languageService.getProgram() === languageService.getProgram())

Expected behavior:

Same Program instance is returned because isProgramUpToDate() returns true.

Actual behavior:

New Program instance is returned each time because isProgramUpToDate() always returns false.

I believe this is the culprit:
https://github.com/microsoft/TypeScript/blob/master/src/compiler/program.ts#L586-L589
It asks HostCache for the version of all program.getSourceFiles().
However, the HostCache only contains host.getScriptFileNames()

host.getScriptFileNames() contains only root files, for example, the contents of a src directory.
program.getSourceFiles() contains things like typescript/lib/lib.es5.d.ts
As far as I know, host.getScriptFileNames() must never include typescript/lib/lib*.d.ts

Thus isProgramUpToDate() is doomed to fail every time, as far as I can tell.

Related Issues:

#36010