fixes #23973; fixes #23974; Memory corruption with lent and ORC (#23981)

fixes #23973;
fixes #23974

compiler/varpartitions.nim

@@ -441,11 +441,15 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
 
   of nkIfStmt, nkIfExpr:
     for i in 0..<n.len:
+      inc c.inConditional
       destMightOwn(c, dest, n[i].lastSon)
+      dec c.inConditional
 
   of nkCaseStmt:
     for i in 1..<n.len:
+      inc c.inConditional
       destMightOwn(c, dest, n[i].lastSon)
+      dec c.inConditional
 
   of nkStmtList, nkStmtListExpr:
     if n.len > 0:
@@ -498,8 +502,17 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
         # we know the result is derived from the first argument:
         var roots: seq[(PSym, int)] = @[]
         allRoots(n[1], roots, RootEscapes)
-        for r in roots:
-          connect(c, dest.sym, r[0], n[1].info)
+        if roots.len == 0 and c.inConditional > 0:
+          # when in a conditional expression,
+          # to ensure that the first argument isn't outlived
+          # by the lvalue, we need find the root, otherwise
+          # it is probably a local temporary
+          # (e.g. a return value from a call),
+          # we should prevent cursorfication
+          dest.flags.incl preventCursor
+        else:
+          for r in roots:
+            connect(c, dest.sym, r[0], n[1].info)
 
       else:
         let magic = if n[0].kind == nkSym: n[0].sym.magic else: mNone

tests/arc/tarcmisc.nim

@@ -32,6 +32,7 @@ true
 copying
 123
 42
+@["", "d", ""]
 ok
 destroying variable: 20
 destroying variable: 10
@@ -789,3 +790,33 @@ block: # bug #23907
 
   doAssert callback(Thingy(value: 123)) == 123
 
+import std/strutils
+
+block: # bug #23974
+  func g(e: seq[string]): lent seq[string] = result = e
+  proc k(f: string): seq[string] = f.split("/")
+  proc n() =
+    const r = "/d/"
+    let t =
+      if true:
+        k(r).g()
+      else:
+        k("/" & r).g()
+    echo t
+
+  n()
+
+block: # bug #23973
+  func g(e: seq[string]): lent seq[string] = result = e
+  proc k(f: string): seq[string] = f.split("/")
+  proc n() =
+    const r = "/test/empty"  # or "/test/empty/1"
+    let a = k(r).g()
+    let t =
+      if true:
+        k(r).g()
+      else:
+        k("/" & r).g()   # or raiseAssert ""
+    doAssert t == a
+
+  n()