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

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

[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:

[bluepichu]

Workaround to the “symbol-as-key” variant of this issue: if you give TS some non-unique symbol way of naming the type, it seems to work out ok. It’s a bit clunky since you have to explicitly type the result, but at least it compiles!

Failing Pattern

// a.ts
export const A = Symbol("A");

export function create() {
	return { [A]: true };
}

// b.ts
import { create } from "./a";
export const b = create(); // TS4023: Exported variable 'b' has or is using name 'A' from external module...

Workaround Pattern

// a.ts
export const A = Symbol("A");

export interface WithAField<T> {
	[A]: T;
}

export function create(): WithAField<true> {
	return { [A]: true };
}

// b.ts
import { create } from "./a";
export const b = create(); // Emits as import("./a").WithAField<true>