From 7c2ac988805111df999dd4f586471627ee80efbb Mon Sep 17 00:00:00 2001
From: Saem Ghani <saemghani+github@gmail.com>
Date: Tue, 23 Feb 2021 00:02:06 -0800
Subject: [PATCH] Fixes #17039 - ldObj checks node/nodeAddr access (#17123)

Checked field expressions, such as an object variant field access results in
occasionally broken address analysis crashing the VM. This guard added here
mimics guarded access in ldObjAddr as well. This is to prevent a crash, while a
fix is devised.
---
 compiler/vm.nim     |  2 +-
 tests/vm/t17039.nim | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 tests/vm/t17039.nim

diff --git a/compiler/vm.nim b/compiler/vm.nim
index 246d9d4d23..ba14f2d51b 100644
--- a/compiler/vm.nim
+++ b/compiler/vm.nim
@@ -711,7 +711,7 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
     of opcLdObj:
       # a = b.c
       decodeBC(rkNode)
-      let src = regs[rb].node
+      let src = if regs[rb].kind == rkNode: regs[rb].node else: regs[rb].nodeAddr[]
       case src.kind
       of nkEmpty..nkNilLit:
         # for nkPtrLit, this could be supported in the future, use something like:
diff --git a/tests/vm/t17039.nim b/tests/vm/t17039.nim
new file mode 100644
index 0000000000..f92c93f30a
--- /dev/null
+++ b/tests/vm/t17039.nim
@@ -0,0 +1,10 @@
+type
+  Obj1 = object
+    case kind: bool
+    of false:
+      field: seq[int]
+    else: discard
+
+static:
+  var obj1 = Obj1()
+  obj1.field.add(@[])