Type error with Typescript 4.8

[fcollonval]

The following template parameter is throwing an error with TypeScript 4.8 because the template of Conditional type is restricted to a subset Conditional<CD extends FieldDef<any> | DatumDef | ValueDef<any> | ExprRef | SignalRef>

vega-lite/src/channeldef.ts

Line 158 in c97e30a

export function isConditionalParameter<T>(c: Conditional<T>): c is ConditionalParameter<T> {

The error raised is

  ../node_modules/vega-lite/build/src/channeldef.d.ts(63,66): error TS2344: Type 'T' does not satisfy the constraint 'SignalRef | ExprRef | FieldDef<any, any> | DatumDef<string, SignalRef | DateTime | ExprRef | PrimitiveValue> | ValueDef<...>'.
    Type 'T' is not assignable to type 'DatumDef<string, SignalRef | DateTime | ExprRef | PrimitiveValue>'.

Seen in https://github.com/jupyterlab/jupyterlab/actions/runs/3217544851/jobs/5260639447

[domoritz]

Is this the same as #8454?

[fcollonval]

Yes it is - but using compiler option is not an option. The code really requires the fixed mentioned in #8454 log:

63 export declare function isConditionalParameter<T>(c: Conditional<T>): c is ConditionalParameter<T>;
                                                  ~
This type parameter might need an `extends SignalRef | ExprRef | FieldDef<any, any> | DatumDef<string, SignalRef | DateTime | ExprRef | PrimitiveValue> | ValueDef<...>` constraint.

The reason is the type constraint on Conditional<T extends ...> and ConditionalParameter<T extends ...>. So the type parameter in isConditionalParameter function must extend it too.

[domoritz]

I see. If skipLibCheck is not an option for you, do you have cycles to help with fixing the types in Vega-Lite? Does it make sense to track this under #5477 instead of this separate issue?

[fcollonval]

Sure I can open a PR. But I would advice keeping it separated from #5477.