void object fields are now ignored by codegen and fields/fieldPairs iterator

compiler/ccgtypes.nim

@@ -1041,6 +1041,8 @@ proc genObjectFields(m: BModule, typ, origType: PType, n: PNode, expr: Rope;
       else: internalError(m.config, n.info, "genObjectFields(nkRecCase)")
   of nkSym:
     var field = n.sym
+    # Do not produce code for void types
+    if isEmptyType(field.typ): return
     if field.bitsize == 0:
       if field.loc.r == nil: fillObjectFields(m, typ)
       if field.loc.t == nil:

compiler/jsgen.nim

@@ -1502,6 +1502,8 @@ proc createRecordVarAux(p: PProc, rec: PNode, excludedFieldIDs: IntSet, output:
     for i in countup(1, sonsLen(rec) - 1):
       createRecordVarAux(p, lastSon(rec.sons[i]), excludedFieldIDs, output)
   of nkSym:
+    # Do not produce code for void types
+    if isEmptyType(rec.sym.typ): return
     if rec.sym.id notin excludedFieldIDs:
       if output.len > 0: output.add(", ")
       output.addf("$#: ", [mangleName(p.module, rec.sym)])

compiler/semfields.nim

@@ -19,6 +19,9 @@ type
     c: PContext
 
 proc instFieldLoopBody(c: TFieldInstCtx, n: PNode, forLoop: PNode): PNode =
+  if c.field != nil and isEmptyType(c.field.typ):
+    result = newNode(nkEmpty)
+    return
   case n.kind
   of nkEmpty..pred(nkIdent), succ(nkSym)..nkNilLit: result = n
   of nkIdent, nkSym:

lib/system.nim

@@ -2700,6 +2700,7 @@ proc `$`*[T: tuple|object](x: T): string =
       firstElement = false
     else:
       result.add("...")
+      firstElement = false
   when not isNamed:
     if count == 1:
       result.add(",") # $(1,) should print as the semantically legal (1,)

tests/objects/t3734.nim

@@ -0,0 +1,17 @@
+discard """
+output: "i0"
+"""
+
+type
+  Application = object
+      config: void
+      i: int
+      f: void
+
+proc printFields(rec: Application) =
+  for k, v in fieldPairs(rec):
+    echo k, v
+
+var app: Application
+
+printFields(app)