diff -r 4103050978f1 -r efb01b140aa7 ext/pybind11/include/pybind11/common.h
--- a/ext/pybind11/include/pybind11/common.h	Tue Feb 21 18:52:39 2017 +0000
+++ b/ext/pybind11/include/pybind11/common.h	Tue Feb 21 18:53:00 2017 +0000
@@ -558,7 +558,8 @@
 struct nodelete { template <typename T> void operator()(T*) { } };
 
 // overload_cast requires variable templates: C++14 or MSVC 2015 Update 2
-#if defined(PYBIND11_CPP14) || _MSC_FULL_VER >= 190023918
+#if defined(PYBIND11_CPP14) || ( \
+    defined(_MSC_FULL_VER) &&_MSC_FULL_VER >= 190023918)
 #define PYBIND11_OVERLOAD_CAST 1
 
 NAMESPACE_BEGIN(detail)
diff -r 4103050978f1 -r efb01b140aa7 ext/pybind11/include/pybind11/stl.h
--- a/ext/pybind11/include/pybind11/stl.h	Tue Feb 21 18:52:39 2017 +0000
+++ b/ext/pybind11/include/pybind11/stl.h	Tue Feb 21 18:53:00 2017 +0000
@@ -246,7 +246,7 @@
     PYBIND11_TYPE_CASTER(T, _("Optional[") + value_conv::name() + _("]"));
 };
 
-#if PYBIND11_HAS_OPTIONAL
+#ifdef PYBIND11_HAS_OPTIONAL
 template<typename T> struct type_caster<std::optional<T>>
     : public optional_caster<std::optional<T>> {};
 
@@ -254,7 +254,7 @@
     : public void_caster<std::nullopt_t> {};
 #endif
 
-#if PYBIND11_HAS_EXP_OPTIONAL
+#ifdef PYBIND11_HAS_EXP_OPTIONAL
 template<typename T> struct type_caster<std::experimental::optional<T>>
     : public optional_caster<std::experimental::optional<T>> {};