[backport] fixes #23690; SIGSEGV with object variants and RTTI (#23703)

fixes #23690

```nim
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)`.

```nim
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)
```

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/arc/tcaseobj.nim b/tests/arc/tcaseobj.nim
index be1d722ed..3499f5c1e 100644
--- a/tests/arc/tcaseobj.nim
+++ b/tests/arc/tcaseobj.nim
@@ -338,3 +338,29 @@ block:
       doAssert ff.s == 12
 
     mainSync()
+
+import std/sequtils
+
+# bug #23690
+type
+  SomeObj* = object of RootObj
+
+  Item* = object
+    case kind*: 0..1
+    of 0:
+      a*: int
+      b*: SomeObj
+    of 1:
+      c*: string
+
+  ItemExt* = object
+    a*: Item
+    b*: string
+
+proc do1(x: int): seq[(string, Item)] =
+  result = @[("zero", Item(kind: 1, c: "first"))]
+
+proc do2(x: int, e: ItemExt): seq[(string, ItemExt)] =
+  do1(x).map(proc(v: (string, Item)): auto = (v[0], ItemExt(a: v[1], b: e.b)))
+
+doAssert $do2(0, ItemExt(a: Item(kind: 1, c: "second"), b: "third")) == """@[("zero", (a: (kind: 1, c: "first"), b: "third"))]"""