Allow arithmetic in enum definitions

[kerams]

Expanding on #14370 and fsharp/fslang-suggestions#539.

One minor problem are error messages. Consider cases with inconsistent types:

In the latter case, TcExpr seems to insert type casts of some sort (I think), which prevent EvalLiteralExprOrAttribArg from evaluating the expression, so type checking fails before enum case type consistency could be checked and we don't get the nice message in the first picture. I could use TcExprOfUnknownType, but I don't know how to "bind" the returned type to the type variable fieldTy.
Sorted

Also, the parentheses are required, but I think that's fine.

[T-Gro]

Can you please add a test for the diagnostics just like in the screenshot?

[T-Gro]

The () will be required only in the enum case, right?
Just to verify - would be good to know if Fantomas keeps those, otherwise a change would be needed there as well.

[kerams]

The () will be required only in the enum case, right?

Sure, otherwise everything would be broken :). I've only swapped enum case constants for expressions in the parser.

[kerams]

All good now, I think.

[kerams]

I could also take this as an opportunity to adjust error ranges if you want. https://sharplab.io/#v2:DYLgZgzgNAJiDUAfALgTwA4FMAEBBbAvALABQ252i2AGodgIwAypFl2AmnQExA== should only highlight the expression range.

[T-Gro]

I could also take this as an opportunity to adjust error ranges if you want. https://sharplab.io/#v2:DYLgZgzgNAJiDUAfALgTwA4FMAEBBbAvALABQ252i2AGodgIwAypFl2AmnQExA== should only highlight the expression range.

Agree, it makes more sense to use the expression's range only.

[kerams]

Bump