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#include <memory>
 
struct base { virtual ~base() {} };
struct derived : public base {};
 
    std::unique_ptr<base> f() { 
      auto p = std::unique_ptr<derived>(new derived()); // std::make_unique<derived>();
      // p->foo(); 
      // works: return std::move(p); 
      return p;
    }
 
    int main() {
        auto x = f();
        return 0;
    }

prog.cpp: In function ‘std::unique_ptr<base> f()’:
prog.cpp:10:14: error: invalid conversion from ‘std::unique_ptr<derived>’ to ‘std::unique_ptr<derived>&&’ [-fpermissive]
In file included from /usr/include/c++/4.7/memory:86:0,
                 from prog.cpp:1:
/usr/include/c++/4.7/bits/unique_ptr.h:155:2: error:   initializing argument 1 of ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Up, _Ep>&&) [with _Up = derived; _Ep = std::default_delete<derived>; <template-parameter-2-3> = void; _Tp = base; _Dp = std::default_delete<base>]’ [-fpermissive]
prog.cpp:10:14: error: cannot convert ‘p’ from type ‘std::unique_ptr<derived>’ to type ‘std::unique_ptr<derived>&&’
prog.cpp:11:5: warning: control reaches end of non-void function [-Wreturn-type]

Standard output is empty