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..78eb6b4fc1858ccdcb095ce3920ae1dbc3344b8c 100644
--- a/webrtc/base/optional_unittest.cc
+++ b/webrtc/base/optional_unittest.cc
@@ -21,6 +21,43 @@ namespace rtc {
 
 namespace {
 
+struct MyUnprintableType {
+  int value;
+};
+
+struct MyPrintableType {
+  int value;
+};
+
+struct MyOstreamPrintableType {
+  int value;
+};
+
+// Since only one of these two (PrintTo and operator<<) will be picked by gtest
+// for printing, we need to ensure it's fine that the other one is unused. The
+// test itself will ensure the correct one is called.
+void PrintTo(const MyPrintableType& mpt, std::ostream* os)
+    __attribute__((unused));
+
+void PrintTo(const MyPrintableType& mpt, std::ostream* os) {
+  *os << "The value is " << mpt.value;
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         const MyPrintableType& mpt) __attribute__((unused));
+
+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 +777,21 @@ 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})));
+}
+
 }  // namespace rtc