When verbatimModuleSyntax: true, allow an explicit isolatedModules: true

[MuleaneEve]

Bug Report

The new verbatimModuleSyntax implies isolatedModules, which is fine. However, it also prevents us from specifying isolatedModules: true explicitly, which should also be fine.

This is an issue in a Next.js app because its build process will force "isolatedModules: true" to be present. This issue has been reported on their side as well.

So it is currently impossible to upgrade a Next.js app to TypeScript 5.

Existing bug reports

This issue was not taken into consideration:

#53374
#53160

🕗 Version & Regression Information

TypeScript 5.0

🙁 Actual behavior

Type error: Option 'isolatedModules' is redundant and cannot be specified with option 'verbatimModuleSyntax'.

🙂 Expected behavior

verbatimModuleSyntax: true should allow isolatedModules: true.

[fatcerberus]

This is an issue in a Next.js app because its build process will force "isolatedModules: true" to be present.

I sure hope somebody picks up that phone, because I called it!

[GitMurf]

While we are waiting for the fix to be applied to v5.0.4 and released, is there a workaround to get this working? Is the only option to set isolatedModules to false until 5.0.4 comes out? Thanks!

[fatcerberus]

@GitMurf The workaround is to not have isolatedModules in your config at all, since isolatedModules: true is implied by verbatimModuleSyntax: true. Setting it to false is actively contradictory and will not fix this error.

[GitMurf]

@fatcerberus thanks for the quick response! So you mean just remove (or comment out) the isolateModules from the tsconfig all together? And effectively then I will have the same settings via the verbatimModuleSyntax?

[fatcerberus]

@GitMurf Yes. Behavior under verbatimModuleSyntax is a superset of isolatedModules. That's why the current error message says the latter option is redundant.

[nyngwang]

@fatcerberus How did you know it's a superset? (this is sincerely asking as I'm beginner) I came from the docs: https://www.typescriptlang.org/docs/handbook/modules/guides/choosing-compiler-options.html#im-using-a-bundler.