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

@@ -514,21 +514,18 @@ proc genRecordFieldsAux(m: BModule, n: PNode,
           let structName = "_" & mangleRecFieldName(m, n[0].sym) & "_" & $i
           var a = newRopeAppender()
           genRecordFieldsAux(m, k, rectype, check, a, unionPrefix & $structName & ".")
-          # When 'k' is 'void', 'a' is the empty string and we just generate
-          # empty struct. This prevents field access errors when generating
-          # static initializers for the type.
-          # See issue #20699
-          if tfPacked notin rectype.flags:
-            unionBody.add("struct {")
-          else:
-            if hasAttribute in CC[m.config.cCompiler].props:
-              unionBody.add("struct __attribute__((__packed__)){")
+          if a != "":
+            if tfPacked notin rectype.flags:
+              unionBody.add("struct {")
             else:
-              unionBody.addf("#pragma pack(push, 1)$nstruct{", [])
-          unionBody.add(a)
-          unionBody.addf("} $1;$n", [structName])
-          if tfPacked in rectype.flags and hasAttribute notin CC[m.config.cCompiler].props:
-            unionBody.addf("#pragma pack(pop)$n", [])
+              if hasAttribute in CC[m.config.cCompiler].props:
+                unionBody.add("struct __attribute__((__packed__)){")
+              else:
+                unionBody.addf("#pragma pack(push, 1)$nstruct{", [])
+            unionBody.add(a)
+            unionBody.addf("} $1;$n", [structName])
+            if tfPacked in rectype.flags and hasAttribute notin CC[m.config.cCompiler].props:
+              unionBody.addf("#pragma pack(pop)$n", [])
         else:
           genRecordFieldsAux(m, k, rectype, check, unionBody, unionPrefix)
       else: internalError(m.config, "genRecordFieldsAux(record case branch)")

tests/objects/tobject_default_value.nim

@@ -476,20 +476,21 @@ template main {.dirty.} =
 
         foo2()
 
-    block: # issue #20699
-      type
-        Either[A,B] = object
-          case kind:bool
-          of false:
-            b: B
-          of true:
-              a: A
-        O = object of RootRef
 
-      proc oToEither(o:O):Either[O,void] =
-        Either[O,void](kind:true,a: o)
+  block: # issue #20699
+    type
+      Either[A,B] = object
+        case kind:bool
+        of false:
+          b: B
+        of true:
+            a: A
+      O = object of RootRef
 
-      discard oToEither(O())
+    proc oToEither(o:O):Either[O,void] =
+      Either[O,void](kind:true,a: o)
+
+    discard oToEither(O())
 
 static: main()
 main()