Issue a better error message when trying to use a modifier outside of a mapped type

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

Code

export type ITSReadonlyToWriteableArray<T extends readonly any[]> =  Omit<T, keyof any[]> & 
 ITSUnpackedArrayLike<T>[] & {
	-readonly [P in number | 'length']: T[P]
};

export type ITSWriteableArray<T extends readonly any[]> = Omit<T, 'length' | number> & {
	-readonly length: number
}
// => here is error

Expected behavior:

no error, more better error message

Actual behavior:

Error:(96, 12) TS1005: ‘[’ expected.

---

and a small ask (don’t need care this Quesion)

Q: how to list all readonly key

[jcalz]

The -readonly notation is specifically only for mapped types, not for index signatures., despite their similar appearance. This is therefore working as intended and not a bug.

Do you know what “search terms” are? It’s hard to tell if you are intentionally ignoring the template or if there is a language issue.

[dragomirtitian]

@bluelovers This whole post is not an issue but rather a question. Using -readonly is only valid in mapped types. There is no bug here, everything is the intended behavior.

Since you are already using Omit to remove length I am unsure what -readonly is trying to achieve. { length: number } wold be a writable property { readonly length: number } is a read-only property.