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

Start from the TypeScript code in the issue and reproduce the TS1005 error with the shown mapped-type modifier examples. Trace the parser or diagnostic entry point responsible for the '[' expected message; done means invalid modifier placement produces a clearer error message without incorrectly rejecting valid mapped types.