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

webrtc/base/optional.h

Index: webrtc/base/optional.h
diff --git a/webrtc/base/optional.h b/webrtc/base/optional.h
index f5354ee0f25fdb6eabcf6fa367e3db64152681ee..104ba6684900d9ba30645db9aaf3cd0cc0c395e5 100644
--- a/webrtc/base/optional.h
+++ b/webrtc/base/optional.h
@@ -13,6 +13,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <ostream>
 #include <utility>
 
 #include "webrtc/base/array_view.h"
@@ -296,6 +297,19 @@ class Optional final {
   };
 };
 
+// Add a PrintTo function for Optionals for use with gtest, only if
+// there is already a valid operator<< available for the enclosed type.
+// The cast in decltype will ensure PrintTo has a void return type.
+template <typename T>
+auto PrintTo(const Optional<T>& opt, std::ostream* os)

[ossu, 2017/02/16 14:23:48]

Turns out the compiler does clever things that linux_memcheck doesn't like. I'll
have to replicate what value_or() does to get it to stop complaining.

[tommi, 2017/02/17 13:46:23]

Can we call this PrintToForTest?

There's a risk that more and more ostream related things start to creep into the
non-test code base and there is already a precedence for "ForTest" methods,
including a presubmit check that prevents non-test code from calling those
methods.

[ossu, 2017/02/17 13:50:06]

Alas, no, it needs to be called PrintTo for gtest to pick it up, c.f.
https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuid...

+    -> decltype(static_cast<void>(*os << *opt)) {

[ossu, 2017/02/16 13:20:05]

The SFINAE here is maybe not super-obvious if one's not worked with such
constructs before. Should I explain it better?
Essentially, the template will be skipped if *os << *opt isn't a valid
expression, since the function will not have a valid return type. Regardless of
the return type of that expression, I'm forcing the decltype() to be void, if
the expression is at all valid, so the return type will be void.

[kwiberg-webrtc, 2017/02/16 14:07:37]

Yes, more explanation is probably called for. How about documenting what it is
and how it behaves in one paragraph, and explaining the details on how it works
in a second paragraph? (The explanation in your message here was good, I think.)

[ossu, 2017/02/16 14:23:48]

Will do!

+  if (opt) {
+    *os << *opt;

[kwiberg-webrtc, 2017/02/16 14:07:37]

This is test code, so maybe indicate that the value is contained in an optional?

[ossu, 2017/02/16 14:23:48]

I considered that but was unsure if it would be more confusing than not. I think
I'd rather not, if it's not very important. I'll think about it some more,
though.

+  } else {
+    *os << "<empty optional>";
+  }
+}
+
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_OPTIONAL_H_