Bad error message for const variables used in const enums

[riggs]

TypeScript Version: 3.3.0-dev.201xxxxx

Search Terms:
const enum initializer ts2474
Code

// lib.ts
const enum STATIC_KEYS {
    ZERO,
    ONE,
    TWO,

    LAST
}

export const STARTING_NUMBER = STATIC_KEYS.LAST;

// consumer.ts
const enum MORE_STATIC_KEYS {
    ONE = STARTING_NUMBER, // error TS2474; STATIC_KEYS.LAST works as expected
    TWO,
    THREE,
}

Expected behavior:
Code compiles with MORE_STATIC_KEYS.ONE === STATIC_KEYS.LAST.

Actual behavior:
error TS2474: In 'const' enum declarations member initializer must be constant expression.
TS recognizes that STARTING_NUMBER has the type STATIC_KEYS.LAST, but doesn't consider it a 'constant expression.'

If this is expected behavior, clarity in the error message would be appreciated.

Playground Link: link

Related Issues:
Multiple discussions about merging/intersecting of const enums, of which the above is my attempted workaround.

[zhmushan]

const enum will be fully erased after compilation, which means you can't index it by an arbitrary value.

[weswigham]

A const value isn't a constant expression in the sense that error message is indicating, unfortunately. It's trying to say that you need the initializer to be a simple number or string, or the result of arithmetic of those - references to other variables (which may have been calculated using functions and other things we won't precalculate and inline) don't count as "constant" in the same sense. Maybe we need to be more clear here. @DanielRosenwasser ?

[DanielRosenwasser]

const enum member initializers can only contain literal values and other const enums values.

[riggs]

I think I was also confused by this language from the handbook explaining what constitutes a 'constant expression':

2. a reference to previously defined constant enum member (which can originate from a different enum).

At any rate, thanks for clarifying.

[DanielRosenwasser]

Thanks @alisabzevari!