SetOptional/SetRequired/SetReadonly: Fix instantiations with index signatures

[som-sm]

Fixes #1013

• Tweaked the implementation of DistributedPick so that it handles any properly
• Set-* types now use DistributedPick instead of Except/Exclude combination

[som-sm]

Note: The updated implementation of DistributedPick introduces subtle changes in it's behaviour, which might be problematic if the goal is to keep DistributedPick fully aligned with the built-in Pick for non-union types.

1. Behaviour with primitives

// Previous behaviour
type Test = Pick<string, 'toUpperCase'>;
//   ^? type Test = { toUpperCase: () => string }

// Updated behaviour
type Test = Pick<string, 'toUpperCase'>;
//   ^? type Test = string

And this happens because homomorphic mapped types pass through primitives unchanged.

2. Picking number from string indexor

// Previous behaviour
type Test = Pick<{ [k: string]: unknown }, number>;
//   ^? type Test = { [x: number]: unknown; }

// Updated behaviour
type Test = Pick<{ [k: string]: number }, number>;
//   ^? type Test = {}

@sindresorhus If you think this might create issues, then I'd create a separate internal Pick type instead of modifying DistributedPick.

[sindresorhus]

I think it would be better to create an internal pick type for now.

Do you think the new behavior is the correct one / better? If so, we could consider changing it in v5: #450

Alternatively, if both the old and new behavior makes sense in different situations, we could consider adding an option to the type.

[som-sm]

It’s hard to determine which behaviour is better, as these are very rare scenarios.

While we could add an option for this, I doubt it'd be very useful—it might just create confusion. Currently, DistributedPick does what it says, it distributes Pick, but introducing additional changes might not be that obvious.

And IMO, the updated behaviour is better in the 1st case, while the previous behaviour is better in the 2nd, so this definitely needs more discussion.

For now, I'll create an internal Pick.

[sindresorhus]

@som-sm Nice work! You have done a lot of great work here lately. Would you be interested in being added as a maintainer? No worries if not :)

[som-sm]

@sindresorhus Yeah sure, I’d be happy to help as a maintainer and contribute further.

[sindresorhus]

Awesome 🎉