fixes #13481; fixes #22708; disable using union objects in VM (#23362)

fixes #13481;
fixes #22708

Otherwise it gives implicit results or bad codegen

compiler/vmgen.nim

@@ -2050,6 +2050,8 @@ proc genSetConstr(c: PCtx, n: PNode, dest: var TDest) =
       c.freeTemp(a)
 
 proc genObjConstr(c: PCtx, n: PNode, dest: var TDest) =
+  if tfUnion in n.typ.flags: # bug #22708 # bug #13481
+    globalError(c.config, n.info, "object with '{.union.}' pragmas is not supported by VM")
   if dest < 0: dest = c.getTemp(n.typ)
   let t = n.typ.skipTypes(abstractRange+{tyOwned}-{tyTypeDesc})
   if t.kind == tyRef:

tests/vm/tvmmisc.nim

@@ -301,14 +301,6 @@ block: # bug #10815
   const a = P()
   doAssert $a == ""
 
-when defined osx: # xxx bug #13481
-  block:
-    type CharSet {.union.} = object
-      cs: set[char]
-      vs: array[4, uint64]
-    const a = Charset(cs: {'a'..'z'})
-    doAssert a.repr.len > 0
-
 import tables
 
 block: # bug #8007