5.5 Beta - Error assigning const string to enum from another file

[lukpsaxo]

🔎 Search Terms

error 5.5 beta "has a string type, but must have syntactically recognizable string syntax when 'isolatedModules' is enabled"

🕗 Version & Regression Information

• This changed between versions 5.4 and 5.5 beta

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.0-beta&ssl=5&ssc=3&pln=1&pc=1#code/KYDwDg9gTgLgBAYwgOwM7wXAvHARAQVwG4AoE4ZAVwFs4AhOAbxLlbgENtEAaEgXyJA

💻 Code

The playground doesn't reproduce - I guess it only happens when importing from another file

otherFile

export const c = "A";

import * as otherFile from './otherFile';

enum B {
    a = otherFile.c,
};

🙁 Actual behavior

'otherFile.c' has a string type, but must have syntactically recognizable string syntax when 'isolatedModules' is enabled

🙂 Expected behavior

No error like in 5.4 - or a better explanation of what I am doing wrong :/

Additional information about the issue

This seems to have also come up in the new errors report:

#57922

[jakebailey]

Isolated modules means that an emitter only needs to see a single file to know how to emit it. In this case, the error is correct because the type of a impacts whether or not the enum has a reverse mapping or not, and is therefore disallowed as only tsc could know that via analyzing other files (while say, babel, could not).

See also #56153

[lukpsaxo]

Reading the linked issue, I think I get it now.. It just wasn't obvious from the error message - "but must have syntactically recognizable string syntax" - I was thinking somehow the type of the constant was ending up string instead of a specific string e.g. "A"

[fatcerberus]

"Must have syntactically-recognizable string syntax" or in other words "we must be able to tell that this is a string (and not, say, a number) just from looking at the syntax", which is not the case here.