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
3

t.cpp: In constructor 'T::T(Derived&)':
Line 5: error: invalid use of undefined type 'struct Base'
compilation terminated due to -Wfatal-errors.