fixes #12978 (#13012)

compiler/cgen.nim

@@ -362,12 +362,14 @@ proc genObjectInit(p: BProc, section: TCProcSection, t: PType, a: var TLoc,
     linefmt(p, section, "$1.m_type = $2;$n", [r, genTypeInfo(p.module, t, a.lode.info)])
   of frEmbedded:
     if optTinyRtti in p.config.globalOptions:
+      var tmp: TLoc
       if mode == constructRefObj:
-        var n = newNodeIT(nkObjConstr, a.lode.info, t)
-        n.add newNodeIT(nkType, a.lode.info, t)
-        genObjConstr(p, n, a)
+        let objType = t.skipTypes(abstractInst+{tyRef})
+        rawConstExpr(p, newNodeIT(nkType, a.lode.info, objType), tmp)
+        linefmt(p, cpsStmts,
+            "#nimCopyMem((void*)$1, (NIM_CONST void*)&$2, sizeof($3));$n",
+            [rdLoc(a), rdLoc(tmp), getTypeDesc(p.module, objType)])
       else:
-        var tmp: TLoc
         rawConstExpr(p, newNodeIT(nkType, a.lode.info, t), tmp)
         genAssignment(p, a, tmp, {})
     else:

tests/destructor/tcomplexobjconstr.nim

@@ -1,5 +1,6 @@
 discard """
-  output: "true"
+  output: '''true
+OK'''
   cmd: "nim c --gc:arc $file"
 """
 
@@ -31,3 +32,25 @@ assert y.more[2] of MyObject1
 assert y.more[2] of RootObj
 
 echo "true"
+
+# bug #12978
+type
+  Vector2* = object of RootObj
+    x*, y*: float
+
+type
+  Vertex* = ref object
+    point*: Vector2
+
+proc newVertex*(p: Vector2): Vertex =
+  return Vertex(point: p)
+
+proc createVertex*(p: Vector2): Vertex =
+  result = newVertex(p)
+
+proc p =
+  var x = Vector2(x: 1, y: 2)
+  let other = createVertex(x)
+  echo "OK"
+
+p()