allow when in enum types

[timotheecour]

when is allowed inside type Foo = object but not inside type Foo = enum; this proposal is to allow:

type Foo = enum
  foo1
  when bar:
    foo2
  else:
    foo3
  foo4

• eg use case: in nim-lang/Nim@1720bae (part of [superseded] new syntax for lvalue references: byaddr x = expr Nim#13342) I had to add nnkCustomDefSection at the end to avoid breaking bootstrap (which isn't still super satisfactory because we're out of sync with an extra element), whereas I could've instead done, with this feature:

type
  NimNodeKind* = enum
    ...
    when defined(nimHasCustomDefSection):
      nnkCustomDefSection,
    ...

• there are obviously other use cases, eg:

type Foo = enum
  kBaz
  when defined windows:
    kFoo
  kBar

[Varriount]

I like this, the only possible issue is that this can affect the ordinal value of an enum

[timotheecour]

I like this, the only possible issue is that this can affect the ordinal value of an enum

but hopefully ordinal value wouldn't be part of public interface

[Araq]

It's a "natural" feature, yet, I prefer to handle this via a macro. The syntactic and implementation effort would be quite severe.

[timotheecour]

in my use case it was for NimNodeKind which can't be handled with a macro; although this is an edge case though and wouldn't justify a language feature just because of that... still not sure to do in that case...

but generally ya a macro would give maximum flexibility and no language change; feel free to close

[Araq]

feel free to close

Such a rare offering that I can't resist.