Fix pybind11 interoperability with Clang trunk (pybind#1269)

Author: wjakob

include/pybind11/cast.h

@@ -1685,6 +1685,9 @@ template <> inline void cast_safe<void>(object &&) {}
 
 NAMESPACE_END(detail)
 
+template <return_value_policy policy = return_value_policy::automatic_reference>
+tuple make_tuple() { return tuple(0); }
+
 template <return_value_policy policy = return_value_policy::automatic_reference,
           typename... Args> tuple make_tuple(Args&&... args_) {
     constexpr size_t size = sizeof...(Args);

include/pybind11/stl.h

@@ -30,7 +30,8 @@
 #    define PYBIND11_HAS_OPTIONAL 1
 #  endif
 // std::experimental::optional (but not allowed in c++11 mode)
-#  if defined(PYBIND11_CPP14) && __has_include(<experimental/optional>)
+#  if defined(PYBIND11_CPP14) && (__has_include(<experimental/optional>) && \
+                                 !__has_include(<optional>))
 #    include <experimental/optional>
 #    define PYBIND11_HAS_EXP_OPTIONAL 1
 #  endif