NimVM reuses default-value sequences, differing from Nim and from NimVM with explicit defaults

[jrfondren]

Description

The following program with identical code blocks (but for let v. const) results in a != b. This isn't the case when the group and groups variables are explicitly initialized to @[].

let a = block:
  var groups: seq[seq[int]]
  for i in 0 ..< 3:
    var group: seq[int]
    for j in 0 ..< 3:
      group.add j
    groups.add group
  groups

const b = block:
  var groups: seq[seq[int]]
  for i in 0 ..< 3:
    var group: seq[int]
    for j in 0 ..< 3:
      group.add j
    groups.add group
  groups

echo a
echo b

Nim Version

Nim Compiler Version 1.6.10 [Linux: amd64]

and:

Nim Compiler Version 1.7.3 [Linux: amd64]
Compiled at 2022-12-01
Copyright (c) 2006-2022 by Andreas Rumpf

git hash: b36f511

Current Output

@[@[0, 1, 2], @[0, 1, 2], @[0, 1, 2]]
@[@[0, 1, 2], @[0, 1, 2, 0, 1, 2], @[0, 1, 2, 0, 1, 2, 0, 1, 2]]

Expected Output

@[@[0, 1, 2], @[0, 1, 2], @[0, 1, 2]]
@[@[0, 1, 2], @[0, 1, 2], @[0, 1, 2]]

Possible Solution

No response

Additional Information

Workaround: don't trust https://nim-lang.org/docs/manual.html#statements-and-expressions-var-statement in const blocks:

let a = block:
  var groups: seq[seq[int]] = @[]
  for i in 0 ..< 3:
    var group: seq[int] = @[]
    for j in 0 ..< 3:
      group.add j
    groups.add group
  groups

const b = block:
  var groups: seq[seq[int]] = @[]
  for i in 0 ..< 3:
    var group: seq[int] = @[]
    for j in 0 ..< 3:
      group.add j
    groups.add group
  groups

echo a
echo b

[metagn]

Related/same #10938

[ringabout]

Duplicate of #10938