fix(typescript): add missing imports

[apfelbox]

Rollup Plugin Name: TypeScript

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

I have no idea how to test this, as we would need to run the build with both versions of TypeScript and I don't know if sth like this is already set up for this repository).

List any relevant issue numbers:

• Fixes part of [TypeScript] Build broken due to type issues #626

Description

These two imports were missing and fail the TypeScript build of the Typescript rollup plugin with TypeScript 4.0+
Not sure if they failed for ^3.9 as well.

I tested this change with both TypeScript 3.9.7 and 4.0.5 and it works.
Doesn't seem to fix the last issue of #626 though.

[apfelbox]

The second issue is here:

type ElementType<T extends Array<any>> = T extends (infer U)[] ? U : never;
                           ^^^^^^^^^^

type StagedTransformerFactory<T extends TransformerStage> = ElementType<CustomTransformers[T]>;
                                                                        ^^^^^^^^^^^^^^^^^^^^^

Because as every property of CustomTransformers can be undefined, CustomTransformers[T] is the defined type|undefined.

And therefore the type tries to assign undefined to T extends any[], which doesn't work.

There are two possible fixes, but I am (semantically) not sure, which one is the better fit.

Solution 1: allow undefined in ElementType

- type ElementType<T extends Array<any>> = T extends (infer U)[] ? U : never;
+ type ElementType<T extends (Array<any>|undefined)> = T extends (infer U)[] ? U : never;

Solution 2: making all properties required, before fetching them

- type StagedTransformerFactory<T extends TransformerStage> = ElementType<CustomTransformers[T]>;
+ type StagedTransformerFactory<T extends TransformerStage> = ElementType<Required<CustomTransformers>[T]>;

[apfelbox]

If somebody can give me feedback on the question in #633 (comment), I can add it (and we can get this merged)

[shellscape]

@apfelbox I'm probably not as learned as you with TS, but could using ElementType<Partial<CustomTransformers[T]>> work here?

[apfelbox]

@shellscape I don't think that that would work. Partial makes every property in an interface optional (e.g. {test: string} -> {test?: string}), but they already are all optional (they all already have a ?:).

That is the main problem, actually: as they are optional, the type of the index access in CustomTransformers[T] can be undefined, which is then in turn not accepted in the definition of ElementType itself.

[shellscape]

@apfelbox does #647 resolve the issue on your end? Seems to be a possible duplication of effort.

[apfelbox]

@shellscape yes, I the meantime #647 popped up and seems to be fixing the same issues.
However I would argue (and have in an comment over there) that one of the fixes is wrong.