diff --git a/core/fxcrt/BUILD.gn b/core/fxcrt/BUILD.gn
index 09400e8..f91be3c 100644
--- a/core/fxcrt/BUILD.gn
+++ b/core/fxcrt/BUILD.gn
@@ -162,6 +162,13 @@
   }
 }
 
+source_set("test_support") {
+  testonly = true
+  sources = [ "string_test_support.cpp" ]
+  configs += [ "../../:pdfium_strict_config" ]
+  deps = [ ":fxcrt" ]
+}
+
 source_set("unit_test_support") {
   testonly = true
   sources = [
@@ -171,6 +178,7 @@
   configs += [ "../../:pdfium_strict_config" ]
   deps = [
     ":fxcrt",
+    ":test_support",
     "//testing/gtest",
   ]
 }
diff --git a/core/fxcrt/bytestring.h b/core/fxcrt/bytestring.h
index 5f1ed53..8bf13e4 100644
--- a/core/fxcrt/bytestring.h
+++ b/core/fxcrt/bytestring.h
@@ -286,6 +286,13 @@
 std::ostream& operator<<(std::ostream& os, const ByteString& str);
 std::ostream& operator<<(std::ostream& os, ByteStringView str);
 
+// This is declared here for use in gtest-based tests but is defined in a test
+// support target. This should not be used in production code. Just use
+// operator<< from above instead.
+// In some cases, gtest will automatically use operator<< as well, but in this
+// case, it needs PrintTo() because ByteString looks like a container to gtest.
+void PrintTo(const ByteString& str, std::ostream* os);
+
 }  // namespace fxcrt
 
 using ByteString = fxcrt::ByteString;
diff --git a/core/fxcrt/string_test_support.cpp b/core/fxcrt/string_test_support.cpp
new file mode 100644
index 0000000..254e37a
--- /dev/null
+++ b/core/fxcrt/string_test_support.cpp
@@ -0,0 +1,15 @@
+// Copyright 2023 The PDFium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <ostream>
+
+#include "core/fxcrt/bytestring.h"
+
+namespace fxcrt {
+
+void PrintTo(const ByteString& str, std::ostream* os) {
+  *os << str;
+}
+
+}  // namespace fxcrt
diff --git a/fpdfsdk/BUILD.gn b/fpdfsdk/BUILD.gn
index b97d881..b3dff66 100644
--- a/fpdfsdk/BUILD.gn
+++ b/fpdfsdk/BUILD.gn
@@ -159,6 +159,7 @@
     "../core/fpdfapi/font",
     "../core/fpdfapi/page",
     "../core/fpdfapi/parser",
+    "../core/fxcrt:test_support",
     "../core/fxge",
   ]
   pdfium_root_dir = "../"