Why this doesn't compile?

[Shammah Chancellor via Digitalmars-d]

On 2014-10-11 18:10:22 +0000, Adam D. Ruppe said:

> On Saturday, 11 October 2014 at 18:01:41 UTC, Uranuz wrote:
>> When I want to pass generic String by const reference I get an error.
> 
> Strings are passed by reference automatically, so you wouldn't want to 
> double reference them anyway.

That's not entirely true.  They're passed by value unless something 
changed.   By value in that the struct representation is passed.  That 
is to say, the length and a pointer.

> 
>> As far as I understand const should accept both mutable and immutable 
>> data. And there I want to pass it by reference.
> 
> Yes, but since "foo" isn't an lvalue (variable), you can't pass it by 
> ref in D, even if it is const. This differs from C++, but I don't 
> recall the reason, I think there's just too many weird edge cases that 
> D wanted to avoid.
> 
> But in the case of strings, arrays, classes, pointers, and user defined 
> structs that wrap these, you don't need to pass it as ref at all, just 
> use plain const.

That's because string literals are stored in the data segment and are 
immutable rvalues.  You can't have a ref to an rvalue.