I expect this code to compile:
  Optional<TestObject> a;
  bool b = a;

because base::Optional has 
  constexpr explicit operator bool()

However, it produces an error
  // error: no viable conversion from
  // 'Optional<base::(anonymous namespace)::TestObject>' to 'bool'

See https://codereview.chromium.org/2437313002

I don't know why this doesn't work, yet.