const ref parameters and r-value references

[Mark Isaacson via Digitalmars-d-learn]

I'm in the process of learning/practicing D and I noticed 
something that seems peculiar coming from a C++ background:

If I compile and run:
void fun(const ref int x) {
  //Stuff
}

unittest {
  fun(5); //Error! Does not compile
}

I get the specified error in my unit test. I understand that the 
cause is that I've attempted to bind ref to an r-value, what's 
curious is that in C++, the compiler realizes that this is a 
non-issue because of 'const' and just 'makes it work'. Is there a 
rationale behind why D does not do this? Is there a way to write 
'fun' such that it avoids copies but still pledges 
const-correctness while also allowing r-values to be passed in?

Thanks in advance!