[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)
```

(cherry picked from commit 262ff648aa)

compiler/liftdestructors.nim

@@ -156,7 +156,7 @@ proc genWasMovedCall(c: var TLiftCtx; op: PSym; x: PNode): PNode =
   result.add(newSymNode(op))
   result.add genAddr(c, x)
 
-proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool) =
+proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool, enforceWasMoved = false) =
   case n.kind
   of nkSym:
     if c.filterDiscriminator != nil: return
@@ -166,6 +166,8 @@ proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool)
         enforceDefaultOp:
       defaultOp(c, f.typ, body, x.dotField(f), b)
     else:
+      if enforceWasMoved:
+        body.add genBuiltin(c, mWasMoved, "`=wasMoved`", x.dotField(f))
       fillBody(c, f.typ, body, x.dotField(f), b)
   of nkNilLit: discard
   of nkRecCase:
@@ -204,7 +206,7 @@ proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool)
       branch[^1] = newNodeI(nkStmtList, c.info)
 
       fillBodyObj(c, n[i].lastSon, branch[^1], x, y,
-                  enforceDefaultOp = localEnforceDefaultOp)
+                  enforceDefaultOp = localEnforceDefaultOp, enforceWasMoved = c.kind == attachedAsgn)
       if branch[^1].len == 0: inc emptyBranches
       caseStmt.add(branch)
     if emptyBranches != n.len-1:
@@ -215,7 +217,7 @@ proc fillBodyObj(c: var TLiftCtx; n, body, x, y: PNode; enforceDefaultOp: bool)
       fillBodyObj(c, n[0], body, x, y, enforceDefaultOp = false)
     c.filterDiscriminator = oldfilterDiscriminator
   of nkRecList:
-    for t in items(n): fillBodyObj(c, t, body, x, y, enforceDefaultOp)
+    for t in items(n): fillBodyObj(c, t, body, x, y, enforceDefaultOp, enforceWasMoved)
   else:
     illFormedAstLocal(n, c.g.config)
 
@@ -281,6 +283,7 @@ proc fillBodyObjT(c: var TLiftCtx; t: PType, body, x, y: PNode) =
     c.kind = attachedDestructor
     fillBodyObjTImpl(c, t, body, blob, y)
     c.kind = prevKind
+
   else:
     fillBodyObjTImpl(c, t, body, x, y)
 

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"))]"""