From 3575f2bf9ccdabe5fa1fcade086ef0c11adc67b5 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Tue, 4 Apr 2023 18:18:43 +0800
Subject: 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    |  8 +++++---
 tests/objects/t20972.nim | 15 +++++++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)
 create mode 100644 tests/objects/t20972.nim

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],
diff --git a/tests/objects/t20972.nim b/tests/objects/t20972.nim
new file mode 100644
index 000000000..6383dc9b1
--- /dev/null
+++ b/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
-- 
cgit 1.4.1-2-gfad0