Skip boost::variant tests on unsupported compilers and versions of Boost

tests/CMakeLists.txt

@@ -114,8 +114,8 @@ if(PYBIND11_TEST_FILES_EIGEN_I GREATER -1)
   endif()
 endif()
 
-# Optional dependency for some tests
-find_package(Boost)
+# Optional dependency for some tests (boost::variant is only supported with version >= 1.56)
+find_package(Boost 1.56)
 
 # Compile with compiler warnings turned on
 function(pybind11_enable_warnings target_name)

tests/test_stl.cpp

@@ -13,7 +13,7 @@
 // Test with `std::variant` in C++17 mode, or with `boost::variant` in C++11/14
 #if PYBIND11_HAS_VARIANT
 using std::variant;
-#elif PYBIND11_TEST_BOOST
+#elif defined(PYBIND11_TEST_BOOST) && (!defined(_MSC_VER) || _MSC_VER >= 1910)
 #  include <boost/variant.hpp>
 #  define PYBIND11_HAS_VARIANT 1
 using boost::variant;