C++ code - 7 lines

Link:

C++, pasted on Jan 17:

struct Base;
struct Derived;

struct T {
   T(Derived& d) : lol(static_cast<Base>(d)) {};
   Base lol;
};

Output:

1 2	Line 6: error: field 'lol' has incomplete type compilation terminated due to -Wfatal-errors.