Get VS 2015 to use const int definitions

VC++ up to VS 2015 RTM does not require explicit storage allocation for
static const integers declared in classes. VS 2015 Update 1 requires
these storage definitions in some cases. It's unclear exactly what
cases - simple tests work with and without the explicit storage
allocation.

Many previous versions of VC++ have theoretically *allowed* a
definition to supply storage, but tests on VC++ 2013 show that this
doesn't actually work correctly - it leads to duplicate definition
errors in Chromium. So, the change is scoped to VS 2015 only.

This change also updates the generated files to match the new generator.

TL;DR - this change is necessary in order for Chromium to build with
VS 2015 Update 1.

src/google/protobuf/compiler/cpp/cpp_enum.cc

@@ -278,9 +278,9 @@ void EnumGenerator::GenerateMethods(io::Printer* printer) {
   if (descriptor_->containing_type() != NULL) {
     // We need to "define" the static constants which were declared in the
     // header, to give the linker a place to put them.  Or at least the C++
-    // standard says we have to.  MSVC actually insists tha we do _not_ define
-    // them again in the .cc file.
-    printer->Print("#ifndef _MSC_VER\n");
+    // standard says we have to.  MSVC actually insists that we do _not_ define
+    // them again in the .cc file, prior to VC++ 2015.
+    printer->Print("#if !defined(_MSC_VER) || _MSC_VER >= 1900\n");
 
     vars["parent"] = ClassName(descriptor_->containing_type(), false);
     vars["nested_name"] = descriptor_->name();
@@ -297,7 +297,7 @@ void EnumGenerator::GenerateMethods(io::Printer* printer) {
         "const int $parent$::$nested_name$_ARRAYSIZE;\n");
     }
 
-    printer->Print("#endif  // _MSC_VER\n");
+    printer->Print("#endif  // !defined(_MSC_VER) || _MSC_VER >= 1900\n");
   }
 }
 

src/google/protobuf/compiler/cpp/cpp_extension.cc

@@ -155,7 +155,7 @@ void ExtensionGenerator::GenerateDefinition(io::Printer* printer) {
   // Likewise, class members need to declare the field constant variable.
   if (descriptor_->extension_scope() != NULL) {
     printer->Print(vars,
-      "#ifndef _MSC_VER\n"
+      "#if !defined(_MSC_VER) || _MSC_VER >= 1900\n"
       "const int $scope$$constant_name$;\n"
       "#endif\n");
   }

src/google/protobuf/compiler/cpp/cpp_message.cc

@@ -1807,7 +1807,7 @@ GenerateClassMethods(io::Printer* printer) {
   }
 
   // Generate field number constants.
-  printer->Print("#ifndef _MSC_VER\n");
+  printer->Print("#if !defined(_MSC_VER) || _MSC_VER >= 1900\n");
   for (int i = 0; i < descriptor_->field_count(); i++) {
     const FieldDescriptor *field = descriptor_->field(i);
     printer->Print(
@@ -1816,7 +1816,7 @@ GenerateClassMethods(io::Printer* printer) {
       "constant_name", FieldConstantName(field));
   }
   printer->Print(
-    "#endif  // !_MSC_VER\n"
+    "#endif  // !defined(_MSC_VER) || _MSC_VER >= 1900\n"
     "\n");
 
   // Define extension identifiers.

src/google/protobuf/compiler/plugin.pb.cc

@@ -172,11 +172,11 @@ static void MergeFromFail(int line) {
 
 // ===================================================================
 
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) || _MSC_VER >= 1900
 const int CodeGeneratorRequest::kFileToGenerateFieldNumber;
 const int CodeGeneratorRequest::kParameterFieldNumber;
 const int CodeGeneratorRequest::kProtoFileFieldNumber;
-#endif  // !_MSC_VER
+#endif  // !defined(_MSC_VER) || _MSC_VER >= 1900
 
 CodeGeneratorRequest::CodeGeneratorRequest()
   : ::google::protobuf::Message(), _internal_metadata_(NULL) {
@@ -656,11 +656,11 @@ CodeGeneratorRequest::proto_file() const {
 
 // ===================================================================
 
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) || _MSC_VER >= 1900
 const int CodeGeneratorResponse_File::kNameFieldNumber;
 const int CodeGeneratorResponse_File::kInsertionPointFieldNumber;
 const int CodeGeneratorResponse_File::kContentFieldNumber;
-#endif  // !_MSC_VER
+#endif  // !defined(_MSC_VER) || _MSC_VER >= 1900
 
 CodeGeneratorResponse_File::CodeGeneratorResponse_File()
   : ::google::protobuf::Message(), _internal_metadata_(NULL) {
@@ -1021,10 +1021,10 @@ void CodeGeneratorResponse_File::InternalSwap(CodeGeneratorResponse_File* other)
 
 // -------------------------------------------------------------------
 
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) || _MSC_VER >= 1900
 const int CodeGeneratorResponse::kErrorFieldNumber;
 const int CodeGeneratorResponse::kFileFieldNumber;
-#endif  // !_MSC_VER
+#endif  // !defined(_MSC_VER) || _MSC_VER >= 1900
 
 CodeGeneratorResponse::CodeGeneratorResponse()
   : ::google::protobuf::Message(), _internal_metadata_(NULL) {

src/google/protobuf/descriptor.cc

@@ -156,7 +156,7 @@ const char* FileDescriptor::SyntaxName(FileDescriptor::Syntax syntax) {
 
 static const char * const kNonLinkedWeakMessageReplacementName = "google.protobuf.Empty";
 
-#ifndef _MSC_VER  // MSVC doesn't need these and won't even accept them.
+#if !defined(_MSC_VER) || _MSC_VER >= 1900
 const int FieldDescriptor::kMaxNumber;
 const int FieldDescriptor::kFirstReservedNumber;
 const int FieldDescriptor::kLastReservedNumber;

src/google/protobuf/wire_format_lite.cc

@@ -49,8 +49,10 @@ namespace google {
 namespace protobuf {
 namespace internal {
 
-#ifndef _MSC_VER    // MSVC doesn't like definitions of inline constants, GCC
-                    // requires them.
+
+#if !defined(_MSC_VER) || _MSC_VER >= 1900
+// Old version of MSVC doesn't like definitions of inline constants, GCC
+// requires them.
 const int WireFormatLite::kMessageSetItemStartTag;
 const int WireFormatLite::kMessageSetItemEndTag;
 const int WireFormatLite::kMessageSetTypeIdTag;