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#include <iostream>
using namespace std;
#ifdef _MSC_VER
#define __PRETTY_FUNCTION__ __FUNCSIG__
#endif
class Foo
{
public:
  Foo()
  {
    cout << __PRETTY_FUNCTION__ << endl;
  }
  Foo(Foo&&)
  {
    cout << __PRETTY_FUNCTION__ << endl;
  }
  explicit Foo(const Foo&)
  {
    cout << __PRETTY_FUNCTION__ << endl;
  }
 
};
void Bar(Foo & a) {}
int main() {
  const Foo f;
  const Foo &f2 = f;
  cout << "@@@@@@+" << endl;
  auto lambda = [f2, f3(f2)]() mutable {
    Bar(f3); //why f3沒保留const
    Bar(f2); //f2確有
  };
  cout << "@@@@@@-" << endl;
  lambda();
  // your code goes here
  return 0;
}

prog.cpp: In lambda function:
prog.cpp:30:11: error: binding 'const Foo' to reference of type 'Foo&' discards qualifiers
     Bar(f2); //f2確有
           ^
prog.cpp:23:6: note:   initializing argument 1 of 'void Bar(Foo&)'
 void Bar(Foo & a) {}
      ^
prog.cpp: In function 'int main()':
prog.cpp:28:17: error: cannot bind 'const Foo' lvalue to 'Foo&&'
   auto lambda = [f2, f3(f2)]() mutable {
                 ^
prog.cpp:13:3: note:   initializing argument 1 of 'Foo::Foo(Foo&&)'
   Foo(Foo&&)
   ^

Standard output is empty