Constants + Generic causing TS4023 ("but cannot be named")

[arcanis]

TypeScript Version: 3.8, but also the 3.9 beta

Search Terms: ts4023 cannot be named constant

Code

Create a tsconfig with declaration: true. Then, in main.ts:

import {StructType} from './lib';

declare function foo<T>(): T;

export const exportedFooFunc = foo<StructType>();

In lib.ts:

export const SOME_CONSTANT = 'fieldKey';

export type MakeStruct<S> = S;
export type StructType = MakeStruct<{
    readonly [SOME_CONSTANT]: any;
}>;

---

Old instructions (don't repro anymore)

Create a tsconfig with declaration: true. Then, in main.ts:

import {
    ReturnType1,
    ReturnType2,
} from './lib';

export const impl1 = (): ReturnType1 => ({
});

export const impl2 = (): ReturnType2 => ({
});

In lib.ts:

export const SOME_CONSTANT = 'myConstant';

export type WrapType<S> = S;

export type ReturnType1 = WrapType<{
    [SOME_CONSTANT]?: {};
}>;

export type ReturnType2 = {
    [SOME_CONSTANT]?: {};
};

Expected behavior:

The file should be properly compiled.

Actual behavior:

TypeScript reports a diagnostic:

Exported variable 'exportedFooFunc' has or is using name 'SOME_CONSTANT' from external module "/.../lib" but cannot be named.

Playground Link: n/a

Related Issues: n/a

[arcanis]

Interestingly, changing ReturnType1 into:

export type ReturnType1 = WrapType<ReturnType2>;

works, even though it should be exactly the same, except we go through one additional level of indirection. I'm also not sure why, but the generated declaration for main.d.ts then becomes:

import { ReturnType2 } from './lib';
export declare const impl1: () => import("./lib").WrapType<ReturnType2>;
export declare const impl2: () => ReturnType2;

Why did ReturnType1 get inlined whereas ReturnType2 didn't?

[unional]

I'm able to reproduce it without generics.

Repro here: https://github.com/cyberuni/typescript-37888

// types.ts
export const typeSym = 'type'

// null.ts
import { typeSym } from './type'

export const nil = { [typeSym]: 'null' as const }

// optional.ts
import { nil } from './null'

export const optional = { nil } // 'typeSym' cannot be named

I explain it in detail here: