[BUG] Integer enums that don't have explicit values set produce broken types

[daogrady]

Is there an existing issue for this?

• I have searched the existing issues

Nature of Your Project

JavaScript, TypeScript

Current Behavior

When defining an integer enum without explicitly assigning values to the keys, the generated types will contain syntax errors (see repro for a sample model).

These enums can be compiled without issue, although the output looks a bit unexpected:

$ cds repl
> await cds.load('foo.cds')
{
  definitions: {
    Foo: {
      kind: 'type',
      type: 'cds.Integer',
      enum: { a: {}, b: {}, c: {} }
    }
  },
  meta: { creator: 'CDS Compiler v4.5.1', flavor: 'inferred' },
  '$version': '2.0'
}

Expected Behavior

Expected output is not clear either. I would have expected to have the keys enumerated in order, starting from 0 (or 1?). The type should be set accordingly.
Log a fatal error prompting the user to explicitly define values. See comment below. Type could be just number iff we want to make sure the run produces syntactically correct types even in this case.

Steps To Reproduce

1. create a file:

// foo.cds
type Foo: Integer enum { a; b; c; }

2. run:

cds-typer foo.cds

3. check the generated index.ts file and find:

export const Foo = {
  a: undefined,
  b: undefined,
  c: undefined,
} as const;
export type Foo =   // <- syntax error

Environment

- **OS**: Mac
- **Node**: irrelevant
- **npm**: irrelevant
- **cds-typer**: 0.16
- **cds**: irrelevant

|  | https://github.com/<your/repo> |
|:---------------------- | ----------- |
| Node.js                | v19.9.0     |
| @sap/cds               | 7.6.0       |
| @sap/cds-compiler      | 4.5.1       |
| @sap/cds-dk            | -- missing  |
| @sap/cds-dk (global)   | 7.5.1       |
| @sap/eslint-plugin-cds | 2.6.5       |
| @sap/cds-mtxs          | 1.14.0      |
| @cap-js/cds-types      | -- missing  |

Repository Containing a Minimal Reproducible Example

No response

Anything else?

This is probably a rather easily fixable issue by iterating over the entries of integer enums and coalescing the values with their respective index.

https://github.com/cap-js/cds-typer/blob/main/lib/components/enum.js

[daogrady]

After conferring with @bugwelle we came to the conclusion that we should not assume an implicit numbering for missing values at this point in time. The compiler may at some point introduce an implicit numbering that could start with either 0 or 1 and would not be resilient to adding new enum values between existing values. So anything we could use would probably produce breaking changes.

While the compiler currently only produces a warning, this should warrant an error from cds-typer, as this is something we currently simply can not provide definitive types for.

[daogrady]

To consider: duplicate values?