Fixed oneof case accessor to cast to enum for C++.

BUILD

@@ -326,6 +326,7 @@ cc_test(
     ],
     deps = [
         ":test_json_upbprotoreflection",
+        ":test_json_upbproto",
         ":upb_json",
         ":upb_test",
     ],

tests/json/test.proto

@@ -29,6 +29,11 @@ message TestMessage {
   map<string, int32>      map_string_int32 = 23;
   map<string, bool>       map_string_bool = 24;
   map<string, SubMessage> map_string_msg = 25;
+
+  oneof o {
+    int32 oneof_int32 = 26;
+    int64 oneof_int64 = 27;
+  }
 }
 
 message SubMessage {

tests/json/test_json.cc

@@ -4,6 +4,7 @@
  */
 
 #include "tests/json/test.upbdefs.h"
+#include "tests/json/test.upb.h"   // Test that it compiles for C++.
 #include "tests/test_util.h"
 #include "tests/upb_test.h"
 #include "upb/handlers.h"

upbc/generator.cc

@@ -362,7 +362,7 @@ void GenerateMessageInHeader(const protobuf::Descriptor* message, Output& output
         fullname);
     output(
         "UPB_INLINE $0_oneofcases $1_$2_case(const $1* msg) { "
-        "return UPB_FIELD_AT(msg, int, $3); }\n"
+        "return ($0_oneofcases)UPB_FIELD_AT(msg, int32_t, $3); }\n"
         "\n",
         fullname, msgname, oneof->name(),
         GetSizeInit(layout.GetOneofCaseOffset(oneof)));