Inconsistent enforcement of allowed keys on an object when using 'extends'

[AustinGrey]

Inconsistent enforcement of allowed keys on an object when using 'extends'

[RyanCavanaugh]

I don't think there's any way to come up with a consistent set of rules when there's an object that doesn't satisfy the constraint. Once there's a contradiction in the system, nothing's going to "make sense".

[Andarist]

Admittedly, this is a little bit confusing. In the second example, it fails to infer Config since that empty barConfig: {} doesn't satisfy your constraint (Configuration<Targ>). So in this case, it errors on those excess properties as the target type is just Configuration<Targ>. On the other hand, in the first example, it manages to infer it successfully and it can't error there. The point of inference is to infer types more specific than the constraint and that's true about what it inferred there.

[fatcerberus]

Your playground link is broken, just to let you know.

Keep in mind, for future reference, that the excess property check is more of a built-in lint rule than a type check, so in general there's no guarantee that it will be consistently applied; the type rules themselves do allow excess properties, even in non-generic contexts.

Honestly I don't actually know what the expected behaviour here is. I feel, given my understanding of 'extends' that it's likely the right behaviour to not have any error on 'c' in both cases.

I'd be inclined to agree; T extends U is an explicit signal that you accept subtypes of U. It's probably a bug that EPC kicks in here at all.
edit: Andarist explained it above. EPC is done against Configuration<Targ> because generic inference from the object literal itself failed.

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.