Disallowed 0 byte structs.

This is because they are incompatible with C++ and possibly other
languages that make them minimum size 1 (to make sure multiple
such objects don't reside at the same address). Forcing them to size
1 was also not practical, as that is requires updating the logic
of a lot of implementations and thus possibly backwards incompatible.

More here: https://github.com/google/flatbuffers/issues/4122

Change-Id: I2bfdc8597b7cfd2235bb4074bb2ae06f81f8e57d

dart/test/monster_test_my_game.example2_generated.dart

@@ -8,8 +8,8 @@ import 'package:flat_buffers/flat_buffers.dart' as fb;
 
 import 'include_test1_my_game.example2_generated.dart';
 import 'include_test2_my_game.example2_generated.dart';
-import './monster_test_my_game.example_generated.dart' as my_game_example;
 import './monster_test_my_game_generated.dart' as my_game;
+import './monster_test_my_game.example_generated.dart' as my_game_example;
 
 class Monster {
   Monster._(this._bc, this._bcOffset);

dart/test/monster_test_my_game.example_generated.dart

@@ -8,8 +8,8 @@ import 'package:flat_buffers/flat_buffers.dart' as fb;
 
 import 'include_test1_my_game.example_generated.dart';
 import 'include_test2_my_game.example_generated.dart';
-import './monster_test_my_game.example2_generated.dart' as my_game_example2;
 import './monster_test_my_game_generated.dart' as my_game;
+import './monster_test_my_game.example2_generated.dart' as my_game_example2;
 
 class Color {
   final int value;

src/idl_parser.cpp

@@ -1778,21 +1778,24 @@ CheckedError Parser::ParseDecl() {
   EXPECT('{');
   while (token_ != '}') ECHECK(ParseField(*struct_def));
   auto force_align = struct_def->attributes.Lookup("force_align");
-  if (fixed && force_align) {
-    auto align = static_cast<size_t>(atoi(force_align->constant.c_str()));
-    if (force_align->type.base_type != BASE_TYPE_INT ||
-        align < struct_def->minalign || align > FLATBUFFERS_MAX_ALIGNMENT ||
-        align & (align - 1))
-      return Error(
-          "force_align must be a power of two integer ranging from the"
-          "struct\'s natural alignment to " +
-          NumToString(FLATBUFFERS_MAX_ALIGNMENT));
-    struct_def->minalign = align;
+  if (fixed) {
+    if (force_align) {
+      auto align = static_cast<size_t>(atoi(force_align->constant.c_str()));
+      if (force_align->type.base_type != BASE_TYPE_INT ||
+          align < struct_def->minalign || align > FLATBUFFERS_MAX_ALIGNMENT ||
+          align & (align - 1))
+        return Error(
+            "force_align must be a power of two integer ranging from the"
+            "struct\'s natural alignment to " +
+            NumToString(FLATBUFFERS_MAX_ALIGNMENT));
+      struct_def->minalign = align;
+    }
+    if (!struct_def->bytesize) return Error("size 0 structs not allowed");
   }
   struct_def->PadLastField(struct_def->minalign);
   // Check if this is a table that has manual id assignments
   auto &fields = struct_def->fields.vec;
-  if (!struct_def->fixed && fields.size()) {
+  if (!fixed && fields.size()) {
     size_t num_id_fields = 0;
     for (auto it = fields.begin(); it != fields.end(); ++it) {
       if ((*it)->attributes.Lookup("id")) num_id_fields++;

tests/include_test/sub/include_test2.fbs

@@ -5,7 +5,7 @@ namespace MyGame.OtherNameSpace;
 
 enum FromInclude:long { IncludeVal }
 
-struct Unused {}
+struct Unused { a:int; }
 
 table TableB {
   a:TableA;

tests/monster_test.schema.json

@@ -16,6 +16,7 @@
     "MyGame_OtherNameSpace_Unused" : {
       "type" : "object",
       "properties" : {
+        "a" : { "type" : "number" }
       },
       "additionalProperties" : false
     },

tests/test.cpp

@@ -1230,6 +1230,7 @@ void ErrorTest() {
   TestError("enum X:byte (bit_flags) { Y=8 }", "bit flag out");
   TestError("table X { Y:int; } table X {", "datatype already");
   TestError("struct X (force_align: 7) { Y:int; }", "force_align");
+  TestError("struct X {}", "size 0");
   TestError("{}", "no root");
   TestError("table X { Y:byte; } root_type X; { Y:1 } { Y:1 }", "end of file");
   TestError("table X { Y:byte; } root_type X; { Y:1 } table Y{ Z:int }",