g++ compile error

template<class F1, class F2>
F2 operator|(F1 f1, F2 f2) { return [=](unsigned long long x) { return f2(f1(x)); }; };

auto func1 = [](unsigned long long x) { return 2.F*x; };

auto func2 = [](float x) { return (int)x; };

int main(int argc, char *argv[])
{
  auto comp = func1|func2;
}