[TODO] VM: allow const ref objects

compiler/types.nim

@@ -1349,8 +1349,7 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
     elif kind in {skVar, skLet}:
       result = t[1]
   of tyRef:
-    if kind == skConst: result = t
-    else: result = typeAllowedAux(marker, t.lastSon, kind, flags+{taHeap})
+    result = typeAllowedAux(marker, t.lastSon, kind, flags+{taHeap})
   of tyPtr:
     result = typeAllowedAux(marker, t.lastSon, kind, flags+{taHeap})
   of tySet:

tests/vm/tconstrefs.nim

@@ -0,0 +1,21 @@
+import std/json
+
+block: # case ref objects
+  const j = parseJson(""" {"x1":12,"x2":"asdf","x3":[1,2]} """)
+  const x1 = j["x1"].getInt
+  const x2 = j["x3"].to(seq[int])
+  when false:
+    # pending https://github.com/nim-lang/Nim/issues/13081
+    echo j["x1"].getInt
+
+  doAssert x1 == 12
+  doAssert x2 == @[1, 2]
+
+block: # case objects
+  type Foo = ref object
+    x1: int
+    x2: string
+    x3: seq[string]
+  const j1 = Foo(x1: 12, x2: "asdf", x3: @["foo", "bar"])
+  doAssert j1[] == Foo(x1: 12, x2: "asdf", x3: @["foo", "bar"])[]
+  doAssert $j1[] == """(x1: 12, x2: "asdf", x3: @["foo", "bar"])"""