[4.10.0][Regression] failure when type checking with exactOptionalPropertyTypes about type-fest

[sindresorhus]

// @Emiyaaaaa

[Emiyaaaaa]

// @Emiyaaaaa

#802 I was tested that it's working after the revert, we can revert first.

exactOptionalPropertyTypes is false in type-fest's tsconfig, should we also support exactOptionalPropertyTypes: true?

[Emiyaaaaa]

I found some interesting thing:

Seem like Merge<T, T> is {a?: T['a']} and certainty T['a'] is string | undefined,
then string | undefined is not assignable to type 'string' which is a in A, so ts throw an error

So the basic problem is that Merge<T, T> is {a?: T['a']} when T is a generic type, this maybe is a feature or rules of typescript I don't know.

In [email protected], Merge used EnforceOptional to convert undefined to ? and remove undefined, but in fact this is a wrong action and people reported the issue. So I removed EnforceOptional in 4.10.0.

If we need to fix tsc error when exactOptionalPropertyTypes: true, I think we can only resolve bug case-by-case like this

How about you think? @sindresorhus

[sindresorhus]

exactOptionalPropertyTypes is false in type-fest's tsconfig, should we also support exactOptionalPropertyTypes: true?

Yes, we should add it to our config.

[AviVahl]

Thanks! ❤️