Run TS type tests on minimum supported version of TS

[SimenB]

🚀 Feature Proposal

Right now, we verify on CI that Jest can be compiled with an older version of TypeScript (at the time of writing, v4.2). We should also run our type tests against that version.

This task is for adding a CI job installing the same minor version of TS (plus @tsd/typescript) we use for the compile check and run our type tests (in addition to the 4.5 (latest, at the moment of writing) run we have today).

/cc @mrazauskas (as an FYI, not that you should fix this (unless you want to! 😀))

[mrazauskas]

That would be good to have.

Semi-related. If I get it right, the lowest supported Node version is 12.13. Just checked, yesterday tests in CI were running on Node v12.22. Is this intended, because of caret in ^12.13? Hm.. Seemed like that could be any version up from v12.13, or? In this case perhaps CI should run on v12.13? Otherwise the lowest supported is not tested.

[SimenB]

We should add a job for the lowest version of 12, 14 and 16 as well probably, yeah

[mrazauskas]

Just saw you updated verifyOldTs script. The test should blow up, because of this line in the code base (type modifiers is TS 4.5 feature, sorry, I introduced it here):

https://github.com/facebook/jest/blob/268bbed4eadaf9f1a6a5de3af6d3f2688a2868cb/packages/jest-circus/src/legacy-code-todo-rewrite/jestAdapterInit.ts#L10

Installing TS v4.2 locally, build fails pointing at this line.

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