TypeScript: Compiling TS with enum strings for 2-way mapping (the old way) causes invalid declaration file (but compiles ok).

I noticed something seems inconsistent. I have a mapping that was working for mapping file types and extensions, which was VERY convenient, and allowed tracking them as proper types (other than string). The .d.ts file is generated ok, without errors! However, if I take that declaration file and add it to another project it breaks.

TypeScript Version: 2.5 (that is the only version I have that was updated with VS 2017)

Code

enum FileTypesMap {
    FileTypeAExt = <any>".aext",
    FileTypeBExt = <any>".bext"
}
enum FileTypesMap {
    FileTypeCExt = <any>".cext",
    FileTypeDExt = <any>".dext"
}

Expected behavior:

file.d.ts output:

declare enum FileTypesMap {
    FileTypeAExt = <any>".aext",
    FileTypeBExt = <any>".bext"
}
declare enum FileTypesMap {
    FileTypeCExt = <any>".cext",
    FileTypeDExt = <any>".dext"
}

(or similar, doesn’t matter)

Actual behavior:

file.d.ts output:

declare enum FileTypesMap {
    FileTypeAExt,
    FileTypeBExt,
}
declare enum FileTypesMap {
    FileTypeCExt,
    FileTypeDExt,
}

ERROR: In an enum with multiple declarations, only one declaration can omit an initializer for its first enum element.

Yes, I am well aware of the issues of enum { A = <any>'B', 'B' = <any>'A' }, and that’s besides the point. If it compiles correctly, the d.ts file should also be proper - it should not be complaining about implementation stuff. 😉

I’m also aware of string enums, and that is also not the point of this code. The point is a mapping between a file type name and an extension (both ways). There are other ways I’m sure, which is also not the point since this should work. Keep in mind this is part of a framework that also allows others to extend the file type names and extensions. I imagine there are other ways, but if this way compiles, again, the declaration should not be complaining.