Fix typeSym.getImpl for ref types (#13752)

* Fix typeSym.getImpl for ref types

* Fix a codegen issue affecting the test suite of nim-beacon-chain

* Fix tests/stdlib/tjsonmacro

To understand the fix better it may help to take a look
at the history of the replaced code.

The nil check that is removed in this commit was introduced
in another fix that failed to identify the root cause of the
issue - namely that we allow an object type to exist for which
no ast is present:

https://github.com/nim-lang/Nim/pull/9601/files

The original intention of the code is more obvious here:

https://github.com/nim-lang/Nim/pull/9538/files

tests/macros/tmacrotypes.nim

@@ -4,11 +4,40 @@ void; ntyVoid; void; void
 int; ntyInt; int; int
 proc (); ntyProc; proc[void]; proc ()
 voidProc; ntyProc; proc[void]; proc ()
+listing fields for ObjType
+a: string
+b: int
+listing fields for ObjRef
+skipping ref type
+a: string
+b: int
+listing fields for RefType
+skipping ref type
+a: int
+b: float
+listing fields for typeof(a)
+skipping ref type
+a: string
+b: int
+listing fields for typeof(b)
+skipping ref type
+a: string
+b: int
+listing fields for typeof(c)
+skipping ref type
+a: int
+b: float
+listing fields for typeof(x)
+a: string
+b: int
+listing fields for typeof(x)
+a: int
+b: float
 typeDesc[range[1 .. 5]]; ntyTypeDesc; typeDesc[range[1, 5]]; typeDesc[range[1 .. 5]]
 typeDesc[range]; ntyTypeDesc; typeDesc[range[T]]; typeDesc[range]'''
 """
 
-import macros
+import macros, typetraits
 
 macro checkType(ex: typed): untyped =
   echo ex.getTypeInst.repr, "; ", ex.typeKind, "; ", ex.getType.repr, "; ", ex.getTypeImpl.repr
@@ -17,7 +46,6 @@ macro checkProcType(fn: typed): untyped =
   let fn_sym = if fn.kind == nnkProcDef: fn[0] else: fn
   echo fn_sym, "; ", fn_sym.typeKind, "; ", fn_sym.getType.repr, "; ", fn_sym.getTypeImpl.repr
 
-
 proc voidProc = echo "hello"
 proc intProc(a: int, b: float): int {.checkProcType.} = 10
 
@@ -26,6 +54,58 @@ checkType(intProc(10, 20.0))
 checkType(voidProc)
 checkProcType(voidProc)
 
+macro listFields(T: typed) =
+  echo "listing fields for ", repr(T)
+  let inputExprType = getType(T)
+
+  var objType = inputExprType[1]
+  if objType.kind == nnkBracketExpr and objType.len > 1:
+    if ((objType[0].kind == nnkRefTy) or
+        (objType[0].kind == nnkSym and eqIdent(objType[0], "ref"))):
+      echo "skipping ref type"
+      objType = objType[1]
+
+  let typeAst = objType.getImpl
+
+  var objectDef = typeAst[2]
+  if objectDef.kind == nnkRefTy:
+    objectDef = objectDef[0]
+
+  let recList = objectDef[2]
+  for rec in recList:
+    echo $rec[0], ": ", $rec[1]
+
+type
+  ObjType* = object of RootObj
+    a: string
+    b: int
+
+  ObjRef = ref ObjType
+
+  RefType* = ref object of RootObj
+    a: int
+    b: float
+
+listFields ObjType
+listFields ObjRef
+listFields RefType
+
+let
+  a = new ObjType
+  b = new ObjRef
+  c = new RefType
+
+listFields typeOf(a)
+listFields typeOf(b)
+listFields typeOf(c)
+
+proc genericProc(x: object) =
+  listFields typeOf(x)
+
+genericProc a[]
+genericProc b[]
+genericProc c[]
+
 # bug #10548
 block:
   var c {.compileTime.} = 0