ARC/nimSeqsV2 breaks seq at compile-time (macro)

[codehz]

Example

macro Bug() =
  let a = @[123]
  discard @[456] & a
  let c = @[789] & a
  assert c[1] == 123

Bug()

command line: nim check --gc:arc a.nim

Current Output

Hint: used config file 'C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\config\nim.cfg' [Conf]
Hint: used config file 'C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\config\config.nims' [Conf]
....
stack trace: (most recent call last)
D:\Temp\nimbug\b.nim(5, 11) Bug
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\assertions.nim(30, 26) failedAssertImpl
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\assertions.nim(23, 11) raiseAssert
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\fatal.nim(49, 5) sysFatal
D:\Temp\nimbug\b.nim(7, 4) template/generic instantiation of `Bug` from here
C:\Users\CodeHz\.choosenim\toolchains\nim-1.4.2\lib\system\fatal.nim(49, 5) Error: unhandled exception: D:\Temp\nimbug\b.nim(5, 10) `c[1] == 123`  [AssertionDefect]

Expected Output

should no error at compile-time

Possible Solution

replace seq with another user-defined list

Additional Information

$ nim -v
Nim Compiler Version 1.4.2 [Linux: amd64]
Compiled at 2020-12-20
Copyright (c) 2006-2020 by Andreas Rumpf

active boot switches: -d:release -d:danger

[ghost]

Superseded by #16847 (same issue, but a bit more explanation)