Add a PrintTo function for rtc::Optional to aid with testing.

webrtc/base/optional_unittest.cc

Index: webrtc/base/optional_unittest.cc
diff --git a/webrtc/base/optional_unittest.cc b/webrtc/base/optional_unittest.cc
index 65070fabb03f2ed0383d64a3aa70f279188e5c14..cc9d2f91643a53aac1a2d5dde7835c7d30b6e7be 100644
--- a/webrtc/base/optional_unittest.cc
+++ b/webrtc/base/optional_unittest.cc
@@ -21,6 +21,34 @@ namespace rtc {
 
 namespace {
 
+struct MyUnprintableType {
+  int value;
+};
+
+struct MyPrintableType {
+  int value;
+};
+
+struct MyOstreamPrintableType {
+  int value;
+};
+
+void PrintTo(const MyPrintableType& mpt, std::ostream* os) {
+  *os << "The value is " << mpt.value;
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         const MyPrintableType& mpt) {
+  os << mpt.value;
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         const MyOstreamPrintableType& mpt) {
+  os << mpt.value;
+  return os;
+}
+
 // Class whose instances logs various method calls (constructor, destructor,
 // etc.). Each instance has a unique ID (a simple global sequence number) and
 // an origin ID. When a copy is made, the new object gets a fresh ID but copies
@@ -740,4 +768,32 @@ TEST(OptionalTest, TestMoveValue) {
       *log);
 }
 
+TEST(OptionalTest, TestPrintTo) {
+  constexpr char kEmptyOptionalMessage[] = "<empty optional>";
+  const Optional<MyUnprintableType> empty_unprintable;
+  const Optional<MyPrintableType> empty_printable;
+  const Optional<MyOstreamPrintableType> empty_ostream_printable;
+  EXPECT_EQ(kEmptyOptionalMessage, ::testing::PrintToString(empty_unprintable));
+  EXPECT_EQ(kEmptyOptionalMessage, ::testing::PrintToString(empty_printable));
+  EXPECT_EQ(kEmptyOptionalMessage,
+            ::testing::PrintToString(empty_ostream_printable));
+  EXPECT_NE("1", ::testing::PrintToString(Optional<MyUnprintableType>({1})));
+  EXPECT_NE("1", ::testing::PrintToString(Optional<MyPrintableType>({1})));
+  EXPECT_EQ("The value is 1",
+            ::testing::PrintToString(Optional<MyPrintableType>({1})));
+  EXPECT_EQ("1",
+            ::testing::PrintToString(Optional<MyOstreamPrintableType>({1})));
+}
+
+void UnusedFunctionWorkaround() {
+  // These are here to ensure we don't get warnings about ostream and PrintTo
+  // for MyPrintableType never getting called.
+  const MyPrintableType dont_warn{17};
+  const MyOstreamPrintableType dont_warn2{18};
+  std::stringstream sstr;
+  sstr << dont_warn;
+  PrintTo(dont_warn, &sstr);
+  sstr << dont_warn2;
+}
+
 }  // namespace rtc