Right now, explicit conversion operators aren't allowed since VC++2013 doesn't actually enforce the explicit part. But on Windows, maybe we have sufficient coverage from the clang-cl builds to be OK with using this?

Even given the VC++2013 limitations, I think we should still consider transitioning now: the current idiom that smart pointers use to make themselves testable allows them to convert to bools in places where a contextual conversion to bool wouldn't normally apply. In the process of migrating from scoped_ptr to std::unique_ptr, we've already found a few bugs:

Example 1:
void BoundBoolSetFromScopedPtrFreeDeleter(
    bool* var,
    scoped_ptr<bool, base::FreeDeleter> val) {
  *var = val;  // should be *val!
}

Example 2:
bool HidReceiveFunction::ValidateParameters() {
  parameters_ = hid::Receive::Params::Create(*args_);
  // Function is supposed to return true if parameters are OK.
  // But this macro returns a non-null scoped_ptr on validation
  // failure, which converts to true, which is the opposite of
  // what we want!
  EXTENSION_FUNCTION_VALIDATE(parameters_);
  ...
}

(I hear VC++2015 is around the corner, so maybe the VC++2013 compat question won't matter for too much longer)