meaning of "auto ref const"?

[Ali Çehreli via Digitalmars-d-learn]

As a general rule, 'auto ref' should probably be const. If the purpose 
of 'ref' is so that the argument would be mutated, then allowing a copy 
of an rvalue to this function could very well be a bug:

struct S {
     int i;
}

void foo()(auto ref S s) {
     s.i = 42;  // <-- Cannot be observed if the arg is rvalue
}

void main() {
     foo(S(1));
}

To contradict myself (and I hate when I do that! :p), the function may 
be using the rvalue in a non-const context, which would make the 
mutation observable:

struct S {
     int i;

     void sayIt() {
         import std.stdio;
         writeln(i);
     }
}

void foo()(auto ref S s) {
     s.i = 42;
     s.sayIt();    // <-- Here
}

// ...

Another one through the return value (but this time it's a copy anyway, 
perhaps defeating the 'ref' purpose):

// ...

S foo()(auto ref S s) {
     s.i = 42;
     return s;
}

void main() {
     foo(S(1)).sayIt();  // <-- Here
}

Ali