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();
}
```

Start with libs/local_function/test/return_assign.cpp and run the existing test to compare the temporary and local-variable cases. Trace how the returned boost::function retains the bound reference, then determine the intended safe test behavior; done means the example no longer relies on a dangling reference and the relevant lifetime case is correctly covered.

Scritto dal modello di indicizzazione a partire dal testo della issue.