Mixin declaration in generic does not work

[Araq]

This should compile, but doesn't

proc enumToString*(enums: openArray[enum]): string =
  mixin e
  when e.low.ord >= 0 and e.high.ord < 256:
    result = newStringOfCap(enums.len)
  else:
    result = newStringOfCap(enums.len * 2)
  for e in enums:
    result.add(e.enumToString)

[mratsim]

Related to #8677 [Meta] Generics/Static early symbol resolution

[mratsim]

Fixing #8603 with f7c0e20 broke Arraymancer tensor concatenation due to the introduction of proc `|`*(a, b: typedesc): typedesc = discard in system.nim.

I fixed it by introducing mixin `|` in a generic proc via mratsim/Arraymancer@a4b79c8 so it actually works.

However the symbol still need to be forward declared (even if in another module) contrary to this example.