Sync from Piper @359788468 PROTOBUF_SYNC_PIPER

diff --git a/conformance/binary_json_conformance_suite.cc b/conformance/binary_json_conformance_suite.cc
index 4a0d8e8..48bfa96 100644
--- a/conformance/binary_json_conformance_suite.cc
+++ b/conformance/binary_json_conformance_suite.cc

@@ -300,7 +300,7 @@
 }
 
 string UpperCase(string str) {
-  for (int i = 0; i < str.size(); i++) {
+  for (size_t i = 0; i < str.size(); i++) {
     str[i] = toupper(str[i]);
   }
   return str;

diff --git a/conformance/conformance_cpp.cc b/conformance/conformance_cpp.cc
index 9546518..d27612b 100644
--- a/conformance/conformance_cpp.cc
+++ b/conformance/conformance_cpp.cc

@@ -55,6 +55,7 @@
 using google::protobuf::util::NewTypeResolverForDescriptorPool;
 using google::protobuf::util::TypeResolver;
 using protobuf_test_messages::proto3::TestAllTypesProto3;
+using protobuf_test_messages::proto2::TestAllTypesProto2;
 using std::string;
 
 static const char kTypeUrlPrefix[] = "type.googleapis.com";
@@ -102,6 +103,8 @@
 
 void DoTest(const ConformanceRequest& request, ConformanceResponse* response) {
   Message *test_message;
+  google::protobuf::LinkMessageReflection<TestAllTypesProto2>();
+  google::protobuf::LinkMessageReflection<TestAllTypesProto3>();
   const Descriptor *descriptor = DescriptorPool::generated_pool()->FindMessageTypeByName(
       request.message_type());
   if (!descriptor) {

diff --git a/conformance/conformance_test_runner.cc b/conformance/conformance_test_runner.cc
index 9f893cb..1572ac0 100644
--- a/conformance/conformance_test_runner.cc
+++ b/conformance/conformance_test_runner.cc

@@ -297,7 +297,7 @@
 
     std::vector<const char *> argv;
     argv.push_back(executable.get());
-    for (int i = 0; i < executable_args_.size(); ++i) {
+    for (size_t i = 0; i < executable_args_.size(); ++i) {
       argv.push_back(executable_args_[i].c_str());
     }
     argv.push_back(nullptr);
@@ -307,7 +307,7 @@
 }
 
 void ForkPipeRunner::CheckedWrite(int fd, const void *buf, size_t len) {
-  if (write(fd, buf, len) != len) {
+  if (static_cast<size_t>(write(fd, buf, len)) != len) {
     GOOGLE_LOG(FATAL) << current_test_name_
                << ": error writing to test program: " << strerror(errno);
   }