Revert "fixes #20699; generate an empty struct for void type" (#20806)

* Revert "fixes #20699; generate an empty struct for void type (#20790)"

This reverts commit 8fcb9380f0.

* better fix [backport]

compiler/ccgexprs.nim

@@ -3230,6 +3230,11 @@ proc caseObjDefaultBranch(obj: PNode; branch: Int128): int =
       return i
   assert(false, "unreachable")
 
+proc isEmptyCaseObjectBranch(n: PNode): bool =
+  for it in n:
+    if it.kind == nkSym and not isEmptyType(it.sym.typ): return false
+  return true
+
 proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
                      result: var Rope; count: var int;
                      isConst: bool, info: TLineInfo) =
@@ -3258,7 +3263,7 @@ proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
     let b = lastSon(obj[selectedBranch])
     # designated initilization is the only way to init non first element of unions
     # branches are allowed to have no members (b.len == 0), in this case they don't need initializer
-    if b.kind == nkRecList and b.len > 0:
+    if b.kind == nkRecList and not isEmptyCaseObjectBranch(b):
       result.add "._" & mangleRecFieldName(p.module, obj[0].sym) & "_" & $selectedBranch & " = {"
       getNullValueAux(p, t,  b, constOrNil, result, countB, isConst, info)
       result.add "}"

compiler/ccgtypes.nim

@@ -514,10 +514,7 @@ proc genRecordFieldsAux(m: BModule, n: PNode,
           let structName = "_" & mangleRecFieldName(m, n[0].sym) & "_" & $i
           var a = newRopeAppender()
           genRecordFieldsAux(m, k, rectype, check, a, unionPrefix & $structName & ".")
-          # When 'k' is 'void', 'a' is the empty string and we just generate
+          if a != "":
-          # empty struct. This prevents field access errors when generating
-          # static initializers for the type.
-          # See issue #20699
             if tfPacked notin rectype.flags:
               unionBody.add("struct {")
             else:

tests/objects/tobject_default_value.nim

@@ -476,6 +476,7 @@ template main {.dirty.} =
 
         foo2()
 
+
   block: # issue #20699
     type
       Either[A,B] = object