Difference between reference and const reference as function parameter?

Without the error message, I'm not exactly sure what the compiler might be complaining about, but I can explain the reason logically:

In the line:

bar(foo()); 

The return value of foo() is a temporary A; it is created by the call to foo(), and then destructed as soon as bar() returns. Performing a non-const operation (i.e. an operation that changes the temporary A) doesn't make sense, as the object A is destructed right afterwards.

Looking a little more, this is a virtual dup of this question:

How come a non-const reference cannot bind to a temporary object?

which has an excellent answer.