Code behaves different with and without --gc:arc

[StefanSalewski]

#from combinatorics import combinations

iterator combinations[T](a: openArray[T]; n: int): seq[T] = # stupid fake iterator
  var i = 0
  while i < len(a):
    var h: seq[T]
    h.setLen(0)
    for x in 0 .. n:
      h.add(a[i])
    yield h
    inc(i)

proc main =
  var unp: seq[int]
  block myb:
    unp = @[1, 2, 3]
    for t in unp.combinations(2):
      try:
        for k in t:
          echo k
          assert k != 1
        break myb
      except:
        echo "exception"

main()

$ nim c -r arcbug.nim 
/arcbug.nim; out: /home/stefan/gintrotest/tests/arcbug [SuccessX]
Hint: /home/stefan/gintrotest/tests/arcbug  [Exec]
1
exception
2
2
2

stefan@nuc ~/gintrotest/tests $ nim c --gc:arc -r arcbug.nim 
/arcbug.nim; out: /home/stefan/gintrotest/tests/arcbug [SuccessX]
Hint: /home/stefan/gintrotest/tests/arcbug  [Exec]
1
Traceback (most recent call last)
/home/stefan/gintrotest/tests/arcbug.nim(26) arcbug
/home/stefan/gintrotest/tests/arcbug.nim(21) main
/home/stefan/Nim/lib/system/assertions.nim(27) failedAssertImpl
/home/stefan/Nim/lib/system/assertions.nim(20) raiseAssert
/home/stefan/Nim/lib/system/fatal.nim(45) sysFatal
/home/stefan/Nim/lib/system/fatal.nim(33) sysFatal
Error: unhandled exception: /home/stefan/gintrotest/tests/arcbug.nim(21, 18) `k != 1`  [AssertionError]
Error: execution of an external program failed: '/home/stefan/gintrotest/tests/arcbug '

stefan@nuc ~/gintrotest/tests $ nim -v
Nim Compiler Version 1.1.1 [Linux: amd64]
Compiled at 2020-02-21
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 3667fd9f426eac968c326b3dc9571b7b7e8fef67
active boot switches: -d:release

[timotheecour]

looks like a duplicate of #13585 which was closed

This is the documented behaviour of --goto:exceptions and hence not a bug. I know it's disliked by many but that doesn't make it a bug.

IMO this will cause real problems when migrating to --gc:arc, and prevents code testability based on doAssertRaises + other things I mentioned in that issue, so we should simply have a compiler flag --catchdefects:on/off to control whether or not Defects (like AssectionError, IndexError etc) are allowed to be caught with --goto:exceptions (implied by --gc:arc)

[Araq]

We're adding catchdefects:on/off but closing for now.

[timotheecour]

nim c --gc:arc -r arcbug.nim now works like nim c -r arcbug.nim, removing wontfix label