Value lost when assigning variable to itself

[arnetheduck]

type
  Eth2Digest = object
    data: array[42, byte]

  BlockId* = object
    root*: Eth2Digest

  BlockSlotId* = object
    bid*: BlockId
    slot*: uint64

func init*(T: type BlockSlotId, bid: BlockId, slot: uint64): T =
  debugecho "init ", bid, " ", slot
  BlockSlotId(bid: bid, slot: slot)

var x: BlockSlotId
x.bid.root.data[0] = 42

x =
  if x.slot > 0:
    BlockSlotId.init(x.bid, x.slot)
  else:
    BlockSlotId.init(x.bid, x.slot)
doAssert x.bid.root.data[0] == 42

init (root: (data: [42, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])) 0
/usercode/in.nim(25)     in
/playground/nim/lib/system/assertions.nim(38) failedAssertImpl
/playground/nim/lib/system/assertions.nim(28) raiseAssert
/playground/nim/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /usercode/in.nim(25, 10) `x.bid.root.data[0] == 42`  [AssertionDefect]

nim 1.2.16, 1.6.4

[ynfle]

With the following changes everything works as expected https://play.nim-lang.org/#ix=3Syp

@@ -1,6 +1,6 @@
 type
   Eth2Digest = object
-    data: array[42, byte]
+    data: seq[byte]
 
   BlockId* = object
     root*: Eth2Digest
@@ -14,7 +14,7 @@ func init*(T: type BlockSlotId, bid: BlockId, slot: uint64): T =
   BlockSlotId(bid: bid, slot: slot)
 
 var x: BlockSlotId
-x.bid.root.data[0] = 42
+x.bid.root.data.add 42
 
 x =
   if x.slot > 0:

I wonder if it's related to #16017

[Menduist]

May also be related to #18342
EDIT: works fine without the if, or with orc, so same symptoms

[ringabout]

fixed by #19618

[arnetheduck]

fixed by #19618

can we get a 1.2/1.6 backport? this is a significant issue.

[ringabout]

can we get a 1.2/1.6 backport? this is a significant issue.

Fyi, it has been backported to 1.6: ff81975