VM fix for refs

compiler/vm.nim

@@ -210,8 +210,14 @@ proc putIntoNode(n: var PNode; x: TFullReg) =
   of rkInt: n.intVal = x.intVal
   of rkFloat: n.floatVal = x.floatVal
   of rkNode:
-    if nfIsRef in x.node.flags: n = x.node
-    else: n[] = x.node[]
+    if nfIsRef in x.node.flags:
+      n = x.node
+    else:
+      let destIsRef = nfIsRef in n.flags    
+      n[] = x.node[]
+      # Ref-ness must be kept for the destination
+      if destIsRef:
+        n.flags.incl nfIsRef
   of rkRegisterAddr: putIntoNode(n, x.regAddr[])
   of rkNodeAddr: n[] = x.nodeAddr[][]
 

tests/vm/tref.nim

@@ -0,0 +1,12 @@
+static:
+  var
+    a: ref string
+    b: ref string
+  new a
+
+  a[] = "Hello world"
+  b = a
+
+  b[5] = 'c'
+  doAssert a[] == "Hellocworld"
+  doAssert b[] == "Hellocworld"