Modified pybind support for C++17 to fix issues with Xcode 11

resources/pybind11/include/pybind11/common.h

@@ -19,7 +19,7 @@
 #if !defined(_MSC_VER) && !defined(__INTEL_COMPILER)
 #  if __cplusplus >= 201402L
 #    define PYBIND11_CPP14
-#    if __cplusplus > 201402L /* Temporary: should be updated to >= the final C++17 value once known */
+#    if __cplusplus >= 201703L /* Temporary: should be updated to >= the final C++17 value once known */
 #      define PYBIND11_CPP17
 #    endif
 #  endif

resources/pybind11/include/pybind11/stl.h

@@ -30,7 +30,7 @@
 #    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

resources/pybind11/tools/pybind11Tools.cmake

@@ -21,10 +21,13 @@ include(CMakeParseArguments)
 function(select_cxx_standard)
   if(NOT PYBIND11_CPP_STANDARD)
     if(NOT MSVC)
+      check_cxx_compiler_flag("-std=c++17" HAS_CPP17_FLAG)
       check_cxx_compiler_flag("-std=c++14" HAS_CPP14_FLAG)
       check_cxx_compiler_flag("-std=c++11" HAS_CPP11_FLAG)
 
-      if (HAS_CPP14_FLAG)
+      if (HAS_CPP17_FLAG)
+        set(PYBIND11_CPP_STANDARD -std=c++17)
+      elseif (HAS_CPP14_FLAG)
         set(PYBIND11_CPP_STANDARD -std=c++14)
       elseif (HAS_CPP11_FLAG)
         set(PYBIND11_CPP_STANDARD -std=c++11)