TSLint 6.1 requires TS 3.8 to compile

[adidahiya]

unintentional breaking change caused by #4915

ts.PrivateIdentifier should not be referenced as a type because it leads to build failures like this in TS 3.7:

$ tsc -p ./src

../../node_modules/tslint/lib/language/utils.d.ts:159:72 - error TS2694: Namespace 'ts' has no exported member 'PrivateIdentifier'.

159 export declare function isPrivateIdentifier(node: ts.Node): node is ts.PrivateIdentifier;

                                                                           ~~~~~~~~~~~~~~~~~

https://app.circleci.com/pipelines/github/palantir/blueprint/30/workflows/8eef41b7-2728-43c5-9121-70a3105bace4/jobs/31261/parallel-runs/0/steps/0-104

[JoshuaKGoldberg]

Interesting. Maybe we should add a CI build task that type-checks the lib .d.ts files to avoid this in the future?

[rrdelaney]

Should it be possible to build TSLint with older TypeScript versions? My assumptions with #4915 was that TSLint should run against older versions, but being built with the latest was OK.

[JoshuaKGoldberg]

Ah, the issue here is that if someone is running tsc in an earlier version than 3.8 on a repository that includes tslint in its node-modules/, the ts.PrivateIdentifier will break type checking.

I didn't consider this when reviewing the PR and can send a quick fix to move the isPrivateIdentifier function into the file it's used.

For folks reading this issue, you could also enable TypeScript's --skipLibCheck to disable these errors. But we'll release a new version so you don't have to. 👍

[JoshuaKGoldberg]

🤖 Beep boop! 👉 TSLint is deprecated 👈 and you should switch to typescript-eslint! 🤖

🔒 This issue is being locked to prevent further unnecessary discussions. Thank you! 👋