ReadonlyNonEmptyArray can't be used in a mapped type

[matulek]

🐛 Bug report

Current Behavior

ReadonlyNonEmpty arrays are problematic when used in mapped types.

In the provided example the error message is:

Type 'T[K]' does not satisfy the constraint 'string'.
Type 'T[keyof T]' is not assignable to type 'string'.
Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'string'.
Type 'T[string]' is not assignable to type 'string'.

Expected behavior

There should be no error in the example I provided.

Reproducible example

import { ReadonlyNonEmptyArray } from 'fp-ts/lib/ReadonlyNonEmptyArray';

type MyType<T extends string> = null;
type MappedType<T extends ReadonlyNonEmptyArray<string>> = {
    [K in keyof T]: MyType<T[K]>;
};

the problem is at line 5 with a generic argument T[K].

Suggested solution(s)

It's probably caused by the type intersection with { readonly 0: T } in here:

/**
 * @category model
 * @since 2.5.0
 */
export declare type ReadonlyNonEmptyArray<A> = ReadonlyArray<A> & {
  readonly 0: A
}

I assume that TypeScript finds a possibility of T[string] because of the object-like fragment in the intersection.

Probably the solution is identical to #1830 I've tested it and it works as expected in my scenario:

type ReadonlyNonEmptyArray<T> = readonly [T, ...T[]];

Your environment

Which versions of fp-ts are affected by this issue? Did this work in previous versions of fp-ts?

Certainly my version, I have no idea whether or not it worked in previous versions but the problem is too straightforward to assume that it did.

Software	Version(s)
fp-ts	2.13.1
TypeScript	4.2.2