clang <= 3.6 compatibility.

Author: rwgk

tests/test_class_sh_virtual_py_cpp_mix.cpp

@@ -11,6 +11,10 @@ class Base {
 public:
     virtual ~Base() = default;
     virtual int get() const { return 101; }
+
+    // Some compilers complain about implicitly defined versions of some of the following:
+    Base()             = default;
+    Base(const Base &) = default;
 };
 
 class CppDerived : public Base {