const enum inside a namespace makes it treated as a value

[ghost]

TypeScript Version: nightly (2.5.0-dev.20170719)

Code

declare const N: number;
declare namespace N {
    const enum E {}
}

Expected behavior:

No error. N does not contain any values, so it should be OK to merge it with a value.

Actual behavior:

src/foo.d.ts(1,15): error TS2451: Cannot redeclare block-scoped variable 'N'.
src/foo.d.ts(2,19): error TS2451: Cannot redeclare block-scoped variable 'N'.

[RyanCavanaugh]

We'll still need to block this under --preserveConstEnums

[weswigham]

More generally, then, we believe

const enum Foo {A, B}
declare const Foo: number;

Is allowable?

I think I understand why this isn't actually a bug - I think it's intended behavior: A const enum is a value-meaning symbol, as it is usable in value positions (even if they're replaced at compile time). As such, a namespace containing it must be valid in value positions and so on; and another value declaration cannot exist for the same symbol (or any symbol in that value-meaning chain) unless they are mergable.

[weswigham]

Pretty much the same with #16804, which is a variation on the same issue.

[UselessPickles]

I just ran into this issue today. Here's my vote for "I think this is a bug that should be fixed", although I see the complication.

At compile time, a const enum's value is a "value-meaning symbol" that needs to follow all the same rules as other "value-meaning symbols", but at runtime the enum (and its values) doesn't exist at all.

Seems like a special edge case that, when not using --preserveConstEnums, a const enum's value should not be considered a "value" for the purpose of determining whether a namespace "contains values". But for all other purposes, a const enum's value should still be treated like a "value-meaning symbol".

[jeswin]

I believe I'm facing the same issue here; I just wanted to add another example.

export function createElement() {
  // some implementation...
}

export const h = createElement;

// This works
export namespace createElement {
  export import JSX = JSXTypes;
}

// This does not work.
export declare namespace h {
  export import JSX = JSXTypes;
}

The reason this is needed is because JSX namespace is picked up from createElement (or h) when jsxFactory = "createElement" (or "h").