tsgo not respecting skipLibCheck

[joshcartme]

Steps to reproduce

A repro is available in https://github.com/joshcartme/tsgo-repros/tree/main/redeclare-global

Behavior with typescript@5.9

tsc produces no output. If you set skipLibCheck to false tsc produces the same error as tsgo below.

Behavior with tsgo

tsgo produces:

src/types.ts:4:20 - error TS2430: Interface 'Locale' incorrectly extends interface 'MLocaleExt'.
  Types of property 'weekdays' are incompatible.
    Type '{ (): string[]; (m: Moment, format?: string): string; }' is not assignable to type 'string[]'.

4   export interface Locale extends MLocaleExt {}
                     ~~~~~~


Found 1 error in src/types.ts:4

[joshcartme]

I did some searching and this started happening with the Nov 21 release.

[jakebailey]

This would have been #2134, because we need to be able to issue the errors regardless of the order in which we check the files (or even when the libs were not checked).

Silencing this with skipLibCheck has always been dubious; it's "your" code that is attempting to incorrectly declare something, so if you really want to silence that, you need to // @ts-expect-error or something in your code, not silence an error that was being reported in a strange place.

You very well could have seen this error positioning depending on what your editor does before diagnostics are checked, e.g. if you had semantic tokens enabled and it just so happened to run before the lib files were checked.

[joshcartme]

@jakebailey makes sense! We'll either fix our busted types or expect an error. Thanks