an example/test uses a dangling reference

libs/local_function/test/return_assign.cpp uses a dangling reference:
```c++
#include
#include
#include
#include

//[return_assign
void call1(boost::function f) { BOOST_TEST(f(1) == 5); }
void call0(boost::function f) { BOOST_TEST(f() == 5); }

boost::function linear(const int& slope) {
int BOOST_LOCAL_FUNCTION(const bind& slope,
int x, default 1, int y, default 2) {
return x + slope * y;
} BOOST_LOCAL_FUNCTION_NAME(lin)

boost::function f = lin; // Assign to local variable.
BOOST_TEST(f(1, 2) == 5);

call1(lin); // Pass to other functions.
call0(lin);

return lin; // Return.
}

void call(void) {
boost::function f = linear(2); // create temporary, bind reference to temporary
BOOST_TEST(f(1, 2) == 5); // !!!!!!!!!!!!!!!!!!!!!!!! use the reference
}
//]

int main(void) {
call();
return boost::report_errors();
}
```
changing the target from temporary to local variable can let the test fail:
```c++
#include
#include
#include
#include

//[return_assign
void call1(boost::function f) { BOOST_TEST(f(1) == 5); }
void call0(boost::function f) { BOOST_TEST(f() == 5); }

boost::function linear(const int& slope) {
int BOOST_LOCAL_FUNCTION(const bind & slope,
int x, default 1, int y, default 2) {
return x + slope * y;
} BOOST_LOCAL_FUNCTION_NAME(lin)

boost::function f = lin; // Assign to local variable.
BOOST_TEST(f(1, 2) == 5);

call1(lin); // Pass to other functions.
call0(lin);

return lin; // Return.
}

void call1(void) {
boost::function f = linear(2);
BOOST_TEST(f(1, 2) == 5); // !!!!!!!!!!!!!!!!!!!!!!!! test passed, although dangling reference
}
void call2(void) {
int a = 2;
boost::function f = linear(a);
a = 3;
BOOST_TEST(f(1, 2) == 5); // !!!!!!!!!!!!!!!!!!!!!!!! test failed
}
//]

int main(void) {
call1();
call2();
return boost::report_errors();
}
```

从 libs/local_function/test/return_assign.cpp 开始，运行现有测试，以比较临时对象和局部变量这两种情况。跟踪返回的 boost::function 如何保留绑定的引用，然后确定预期的安全测试行为；完成的标准是该示例不再依赖 dangling reference，并且相关的 Lifetime 情况得到了正确覆盖。

由索引模型根据 Issue 内容生成。