Cannot use const string in enum when it's type is specified explicitly

[undsoft]

🔎 Search Terms

enum string, enum const

🕗 Version & Regression Information

• This is the behavior in every 5+ version available in Playground.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZgQwJYBsIC4YHIAqBTADyixgF5t8isBuAKFElgHcQAnAawjIsOLtrxgArgFsYARSF5oScDgCeABzwwA3rRgxKscolQQANBq28ATNxYdDtAL5A

💻 Code

const fails: 'Text' = 'Text';
const works = 'Text';

enum QuestionType {
  Text = fails,
  Text2 = works,
}

🙁 Actual behavior

When fails is used in enum, there is an error:

Type 'string' is not assignable to type 'number' as required for computed enum member values.(18033)

🙂 Expected behavior

I can use fails, because to me it is equivalent to works.

Additional information about the issue

Related to #29718 ?

[ahejlsberg]

Enum members may be initialized by constant expressions that compute string or numeric values. Constant expressions may reference const variables, but only if those variables have no type annotations. When enum member initializers reference variables with type annotations, they're considered non-constants to be computed at run-time. Such computed enum members are restricted to numeric values. See #50528 for the exact rules.

[RyanCavanaugh]

Constant expressions may reference const variables, but only if those variables have no type annotations

I think this is probably the thing that sounds surprising, but presumably it's to ward off problems like this?

// Correct emit is ... ?
const fails: number | 'Text' = 'Text';

[ahejlsberg]

but presumably it's to ward off problems like this?

That's one reason, but another is to avoid circularities. Imagine that a type annotation depends on the enum type for which we are in the process of evaluating constant expressions.

[Andarist]

Isn't @RyanCavanaugh argument already somewhat trumped by allowing this in other similar situations like this one?

// input
const one: number | "Text" = "Text";
const two = one;

// dts output
declare const one: number | "Text";
declare const two: "Text";

I also found a confusing error to be reported in a related situation:

const a = E.two; // A member initializer in a enum declaration cannot reference members declared after it, including members defined in other enums.(2651)
const b = `pre${a}`;

const enum E {
  one = "foo",
  two = `${E.one}bar`,
}

The location of the error seems to be wrong. It mentions a member initializer but the error node is completely different. On top of that, the referenced enum member doesn't depend on any member declared after it.

This problem goes away if we remove const b completely or if we annotate const a with E.two or similar.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.