[TS 4.1] Array.isArray incorrectly infers recursive type

[victorgarciaesgi]

TypeScript Version: 4.1.2

Code

I have this recursive type that I use for form generation.

export type FormStructure<TForm extends ObjectLiteral> = {
 [TKey in keyof TForm]?: TForm[TKey] extends DefaultFieldStruture
 ? TForm[TKey]
 : TForm[TKey] extends Array<any>
 ? FormStructure<UnArray<TForm[TKey]>>[]
 : FormStructure<TForm[TKey]>;
};

And inside a recursive function, I traverse this big nested object

declare const fields: FormStructure<TForm>

return (Object.entries(fields) as [
 string,
 DefaultFieldStruture | FormStructure<TForm>
 ][]).reduce<TReturn>((acc, [key, field]) => {
 if (field instanceof DefaultFieldStruture) {
 //
 } else if (Array.isArray(field) && Array.isArray(datas[key])) {
 acc[key] = field
 .map((element, index) => {
// ^^^^
 // This expression is not callable.
 // Each member of the union type '(<U>(callbackfn: (value: any, index: number, array: any[]) => U, thisArg?: any) => U[]) | 
 //(<U>(callbackfn: (value: any, index: number, array: readonly any[]) => U, thisArg?: any) => U[])' has signatures, but none 
 // of those signatures are compatible with each other.
 return mapAndCompareThroughData(element, datas[key]?.[index], predicate);
 })
 .filter((result) => !!result);
 } 
 }
 return acc;
 }, {} as TReturn);

Expected behavior:

Before passing into Array.isArray, my field var is correctly infered as

var field: FormStructure<TForm>

After passing thought Array.isArray, it is now

var field: FormStructure<TForm> & (FormStructure<TForm> extends readonly any[] ? unknown extends readonly any[] & FormStructure<TForm> ? never : readonly any[] : any[])

I've tried puting a -readonly to my type but without success

export type FormStructure<TForm extends ObjectLiteral> = {
 -readonly [TKey in keyof TForm]?: TForm[TKey] extends DefaultFieldStruture
 ? TForm[TKey]
 : TForm[TKey] extends Array<any>
 ? FormStructure<UnArray<TForm[TKey]>>[]
 : FormStructure<TForm[TKey]>;
};

Is there a way to make a type guard for this? Why Array.isArray became so strange to analyse?

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-beta#

The playground exemple doesn't works because TS 4.1-beta don't includes isArray new types

[victorgarciaesgi]

Nevermind, found the issue, I replaced

(Object.entries(fields) as [
 string,
 DefaultFieldStruture | FormStructure<TForm>
 ][])

by

(Object.entries(fields) as [
 string,
 DefaultFieldStruture | FormStructure<T> | (FormStructure<T> | DefaultFieldStruture)[]
 ][])

And added extra check in the map interator