fix for https://github.com/nim-lang/Nim/pull/15545

[timotheecour]

fix for nim-lang/Nim#15545 ; it's the only important_package that was broken under nim-lang/Nim#15545

trailing else was implemented here: nim-lang/Nim#14190

[Araq]

This shouldn't have been merged, the PR it refers to wasn't accepted and there is a better way to do this. Besides this breaks "nimpy" for the 1.2.x line.

[narimiran]

@yglukhov Just to let you know that after the changes in this PR, nimpy is now broken on our CIs for Nim 1.2.x and 1.0.x branches.

(EDIT: ah, Araq was faster than me :))

[yglukhov]

I've got no strong opinion about how it should be done. Just tell me if I should revert it :)

[timotheecour]

if this PR is reverted can we backport trailing else feature to 1.0.X and 1.2.X branches for similar reasons as nim-lang/RFCs#214

there is a better way to do this

which one besides making the code more complicated / potentially less efficient / less type-checked with something like:

case expr
of f1: ...
of f2: ...
of f3: ...

=>
template handleF3() = ...
let x = expr
case x
of f1: ...
of f2: ...
else:
  when isOldEnum:
    if x == f3:
      handleF3()
    else: doAssert false # checking whether it's exhaustive is now only RT, not CT
  else:
    case x
    of f3: handleF3()
    of f4: ...
    of f5: ...      
    else: doAssert false # ditto

(done this way to keep code DRY)

(see also nim-lang/Nim#15646 (comment))

[narimiran]

Just tell me if I should revert it :)

yes :)

[yglukhov]

Reverted