`skipLibCheck: false` does not output diagnostics for ambient types

What happens and why it is wrong

tsconfig.compilerOptions.skipLibCheck is by default false, but will always be forced to true when using rpts2.

This isn’t documented in the README “Some compiler options are forced” so I’m not sure why this is the case or if this is intentional. I couldn’t find any code references or issues about this.

Related to https://github.com/jaredpalmer/tsdx/issues/529#issuecomment-593737935

Environment

TSDX@0.12.3

Versions

• typescript: 3.7.3
• rollup: 1.27.8
• rollup-plugin-typescript2: 0.25.3

rollup.config.js

TSDX Rollup

tsconfig.json

package.json

plugin output with verbosity 3

[ezolenko]

Plugin’s exclude is for avoiding transpiling those files (no js would be generated by transpiling d.ts), also typescript files shouldn’t be importing type files, I think they are supposed to be magically found by tsc, but never imported.

Also rollup does tree shaking and some files might be never imported. You should see evidence of that in verbose output – do a build and see if any source files that use affected type file are even transpiled.

Also check if cache is potentially hiding it, do a build with clean: true and see if you see expected error. rpt2 tries to build full import tree when tracking which files should be rebuilt, but ephemeral things like d.ts files might slip through the cracks.

Usually typescript will find all ambient type files and rpt2 will list them at the start of the build and invalidate cache if they changed.

[agilgur5]

So now that I know this codebase really well, I’m pretty sure that the problem is that rpt2 doesn’t output diagnostics for any ambient types. I’ve renamed the issue as such. We basically only output diagnostics in the transform hook, so only for files that are being compiled to JS and not treeshaken by Rollup (i.e. #298)

So while the skipLibCheck setting may impact TS’s actual program analysis, no diagnostics will be output regardless of how it’s set, because we don’t process ambient typings.

Basically, to properly follow TS behavior, we’d want to emit diagnostics for all ambient typings as well when skipLibCheck: false. Possibly in checkAmbientTypes.

That would greatly decrease performance, so I might suggest to add a plugin option that disables this by default. That being said, skipLibCheck is recommended to be false by the TS team anyway:

• Discussion in downstream TSDX: https://github.com/jaredpalmer/tsdx/issues/529#issuecomment-598304924 (discussion includes me)
• @tsconfig/recommended setting
• tsc --init setting: https://github.com/microsoft/TypeScript/pull/37808
• tsconfig docs recommendation: https://github.com/microsoft/TypeScript-Website/pull/970 (PR is by me)

Definitely low priority if it would be disabled by default; that would just be a tsc parity thing then.

That being said, I believe when I originally made this issue I was actually authoring a declaration file and it wasn’t checked (or something? would’ve been great if I made a repro 😅 ). This not being type-checked because of the default exclude is probably accurate in that case and that use-case wouldn’t be so low priority.