Throw error for TypeScript declare const enum

[dosentmatter]

Q	A
Fixed Issues?	Fixes #10785
Patch: Bug Fix?	👍
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	👍
Documentation PR Link
Any Dependency Changes?
License	MIT

In #10785 @fiesh reported that declare const enum was transpiling as if it was declare enum. Since we don't support const enum, an error should be thrown whether it has declare or not.

[nicolo-ribaudo]

I'm not sure if this should be categorized as a breaking change (and thus go in Babel 8) or as a bugfix.

[dosentmatter]

@nicolo-ribaudo, I guess it depends how you interpret the docs.

In docs, we say https://babeljs.io/docs/en/babel-plugin-transform-typescript#caveats

This plugin does not support const enums because those require type information to compile.

We don't specify whether const enums includes declare const enum.

I think const enum and declare const enum are the same thing anyway. declare makes it so no code is generated and it is only a hint to the compiler. It doesn't affect const enum since const enum already generates 0 code.

See:
https://stackoverflow.com/questions/35019987/what-does-declare-do-in-export-declare-class-actions
https://www.typescriptlang.org/docs/handbook/enums.html#ambient-enums

tsc has same output for both:

---

I think it's better to treat it as a bug. Because it seems we just accidentally put the two if (node.const) { and if (node.declare) { in the wrong order.

The current code transpiles declare const enum correctly because it just removes it.

But the problem is with accessors. It's transpiling enum access as if it were declare enum. If you look at #10785, you'll see that

declare const enum Enum {
  A = 1,
}
console.log(Enum.A)

transpiles to

console.log(Enum.A)

If we supported const enum, it should be

console.log(1)

Since we don't transpile the accessors correctly, I think it's better to treat this as a bug. I'm not sure if we could consider the current functionality "working". It doesn't match the TS spec and I don't think we documented it.

If someone did depend on declare const enum working, they can fix it by changing to declare enum, which is the more correct in TypeScript.