[backport] fixes #23690; SIGSEGV with object variants and RTTI

[ringabout]

fixes #23690

dest.`:state` = src.`:state`
var :tmp_553651276 = dest.e1.a
`=wasMoved`(dest.e1.a)
dest.e1.a.kind = src.e1.a.kind
case dest.e1.a.kind
of 0:
  dest.e1.a.a = src.e1.a.a
of 1:
  `=copy`(dest.e1.a.c, src.e1.a.c)
case :tmp_553651276.kind
of 0:
of 1:
  `=destroy`(:tmp_553651276.c)

dest.e1.a.kind = src.e1.a.kind changes the discrimant but it fails to clear the memory of dest.e1.a. Before using hooks for copying, we need to clear the dest, e.g. =wasMoved(dest.e1.a.c).

dest.`:state` = src.`:state`
var :tmp_553651276 = dest.e1.a
`=wasMoved`(dest.e1.a)
dest.e1.a.kind = src.e1.a.kind
case dest.e1.a.kind
of 0:
  `=wasMoved`(dest.e1.a.a)
  dest.e1.a.a = src.e1.a.a
  `=wasMoved`(dest.e1.a.b)
of 1:
  `=wasMoved`(dest.e1.a.c)
  `=copy`(dest.e1.a.c, src.e1.a.c)
case :tmp_553651276.kind
of 0:
of 1:
  `=destroy`(:tmp_553651276.c)

[alex65536]

Thanks for the fix :)

Is it possible to [backport] the commit into stable, by the way?

[github-actions[bot]]

Thanks for your hard work on this PR! The lines below are statistics of the Nim compiler built from 262ff648aad7cc2062eb94cb6bf3357f9910c448

Hint: mm: orc; opt: speed; options: -d:release 178739 lines; 8.284s; 664.652MiB peakmem