Cannot use `const enum` to define an interface key in `isolatedModules` mode

[BenoitZugmeyer]

Bug Report

I am maintaning a library where I use const enum variants to declare the keys of an interface (interface Foo { [MyConstEnum.Foo]: "bar" }). The enum and the interface are in the same module.

It works well, but once compiled using tsc and included in a user project using isolatedModules: true, TypeScript fails with the following error:

Error: [...].d.ts: error TS2748: Cannot access ambient const enums when 'isolatedModules' is enabled.

I don't understand why this usage is restricted, since the enum is only used in a type definition, and both are present in the same .d.ts file.

🔎 Search Terms

Cannot access ambient const enums when the '--isolatedModules' flag is provided. TS2748 interface

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about isolated modules and const enum

⏯ Playground Link

Playground link with relevant code

💻 Code

• Using isolatedModules: true
• In a .d.ts file

declare const enum EventName {
    FOO = 1,
    BAR = 2
}

type EventMap = {
    [EventName.FOO]: number;
    [EventName.BAR]: string;
};

🙁 Actual behavior

TS fails with the following error:

Error: [...].d.ts: error TS2748: Cannot access ambient const enums when 'isolatedModules' is enabled.

🙂 Expected behavior

TS succeeds

Workaround

As a workaround, I declare a fake object using the same keys and values as the enum, and then use this to declare the interface:

declare const enum EventName {
    FOO = 1,
    BAR = 2
}

declare const EventNameAsConst: {
  FOO: EventName.FOO
  BAR: EventName.BAR
}

type EventMap = {
    [EventNameAsConst.FOO]: number;
    [EventNameAsConst.BAR]: string;
};

This works, but it is a bit cumbersome to have to maintain this intermediary object.