fix #20972 fixes invalid and UB codegen case object transitions for both refc and ORC [backport] (#21611)

fix #20972 fixes invalid and UB codegen case object transitions for refc and ORC

1 files changed, 5 insertions, 3 deletions

diff --git a/lib/system/assign.nim b/lib/system/assign.nim
index 42587929c..9f4cbc0fe 100644
--- a/lib/system/assign.nim
+++ b/lib/system/assign.nim
@@ -275,10 +275,12 @@ proc genericReset(dest: pointer, mt: PNimType) =
 
 proc selectBranch(discVal, L: int,
                   a: ptr array[0x7fff, ptr TNimNode]): ptr TNimNode =
-  result = a[L] # a[L] contains the ``else`` part (but may be nil)
   if discVal <% L:
-    let x = a[discVal]
-    if x != nil: result = x
+    result = a[discVal]
+    if result == nil:
+      result = a[L]
+  else:
+    result = a[L] # a[L] contains the ``else`` part (but may be nil)
 
 proc FieldDiscriminantCheck(oldDiscVal, newDiscVal: int,
                             a: ptr array[0x7fff, ptr TNimNode],