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

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],

tests/objects/t20972.nim

@@ -0,0 +1,15 @@
+discard """
+  matrix: "--mm:refc -d:release; --mm:orc -d:release"
+"""
+
+{.passC: "-fsanitize=undefined -fsanitize-undefined-trap-on-error -Wall -Wextra -pedantic -flto".}
+{.passL: "-fsanitize=undefined -fsanitize-undefined-trap-on-error -flto".}
+
+# bug #20972
+type ForkedEpochInfo = object
+  case kind: bool
+  of true, false: discard
+var info = ForkedEpochInfo(kind: true)
+doAssert info.kind
+info.kind = false
+doAssert not info.kind