Use an explicit union with commented explanation rather than casting

PiperOrigin-RevId: 698437576

upb/message/value.h

@@ -15,6 +15,7 @@
 #include <string.h>
 
 #include "upb/base/string_view.h"
+#include "upb/message/internal/types.h"
 
 // Must be last.
 #include "upb/port/def.inc"
@@ -41,6 +42,14 @@ typedef union {
   // documentation in kUpb_DecodeOption_ExperimentalAllowUnlinked for more
   // information.
   uintptr_t tagged_msg_val;  // upb_TaggedMessagePtr
+
+  // For an extension field, we are essentially treating ext->data (a
+  // upb_MessageValue) as if it were a message with one field that lives at
+  // offset 0. This works because upb_MessageValue is precisely one value that
+  // can hold any type of data. Recall that an extension can be of any type
+  // (scalar, repeated, or message). For a message extension, that will be a
+  // single upb_Message* at offset 0 of the upb_MessageValue.
+  struct upb_Message UPB_PRIVATE(ext_msg_val);
 } upb_MessageValue;
 
 UPB_API_INLINE upb_MessageValue upb_MessageValue_Zero(void) {

upb/wire/decode.c

@@ -828,7 +828,7 @@ static void upb_Decoder_AddKnownMessageSetItem(
   }
   upb_Message* submsg = _upb_Decoder_NewSubMessage2(
       d, ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg),
-      &ext->ext->UPB_PRIVATE(field), (upb_TaggedMessagePtr*)&ext->data);
+      &ext->ext->UPB_PRIVATE(field), &ext->data.tagged_msg_val);
   upb_DecodeStatus status = upb_Decode(
       data, size, submsg, upb_MiniTableExtension_GetSubMessage(item_mt),
       d->extreg, d->options, &d->arena);
@@ -1182,7 +1182,7 @@ const char* _upb_Decoder_DecodeKnownField(upb_Decoder* d, const char* ptr,
       _upb_Decoder_ErrorJmp(d, kUpb_DecodeStatus_OutOfMemory);
     }
     d->original_msg = msg;
-    msg = (upb_Message*)&ext->data;
+    msg = &ext->data.UPB_PRIVATE(ext_msg_val);
     if (upb_MiniTableField_IsSubMessage(&ext->ext->UPB_PRIVATE(field))) {
       ext_sub.UPB_PRIVATE(submsg) =
           &ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg);

upb/wire/encode.c

@@ -547,7 +547,7 @@ static void encode_ext(upb_encstate* e, const upb_Extension* ext,
       sub.UPB_PRIVATE(subenum) =
           ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(subenum);
     }
-    encode_field(e, (upb_Message*)&ext->data, &sub,
+    encode_field(e, &ext->data.UPB_PRIVATE(ext_msg_val), &sub,
                  &ext->ext->UPB_PRIVATE(field));
   }
 }