Fix bug in tag overflow checks

internal/tdp/tag.go

@@ -79,5 +79,5 @@ func (t Tag) Format(s fmt.State, verb rune) {
 // Returns whether this tag is "too large", i.e., if it has more than 32 bits
 // when decoded.
 func (t Tag) Overflows() bool {
-	return bits.LeadingZeros64(uint64(t)) < (64 - 32 + 4)
+	return bits.LeadingZeros64(uint64(t)) < (64 - 32 - 4)
 }

internal/tdp/tag_test.go

@@ -0,0 +1,19 @@
+package tdp
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/encoding/protowire"
+)
+
+func TestTagOverflows(t *testing.T) {
+	tag := EncodeTag(protowire.MaxValidNumber, protowire.BytesType)
+	if tag.Overflows() {
+		t.Error("protowire.MaxValidNumber should not overflow")
+	}
+
+	tag = EncodeTag(protowire.MaxValidNumber+1, protowire.BytesType)
+	if !tag.Overflows() {
+		t.Error("protowire.MaxValidNumber+1 should overflow")
+	}
+}