[ref variables] Allow `auto ref`

[Quirin Schroll]

On Tuesday, 30 July 2024 at 05:43:24 UTC, IchorDev wrote:
> On Monday, 29 July 2024 at 15:26:39 UTC, Quirin Schroll wrote:
>> In particular in generic code, `auto ref` variables that infer 
>> `ref` if and only if the initializer is an lvalue could be 
>> quite valuable. The same is true for `foreach` over ranges, 
>> where the foreach variable would infer `ref` if `front` is an 
>> lvalue (be it a variable or a function that returns by 
>> reference) and lastly, for conditions, `if (auto ref x = f())` 
>> could infer `ref` from the value category of the initializer, 
>> like any other variable.
>
> This makes a lot of sense. I’d also love it if `auto ref` 
> worked inside all templates, not just template functions. Often 
> I want to do this:
> ```d
> struct S(T){
>   this(auto ref T x){}
> }
> ```
> But I’d get an error:
> ```d
> S!long x; //Error: cannot explicitly instantiate template 
> function with `auto ref` parameter
> ```
> If I make `this` a template function it works. For template 
> functions, even explicit instantiation works just fine. This 
> requirement feels rather silly and leads to a lot of 
> unnecessary parameter-less templates.

By virtue of how `auto ref` works (and is expected to work), this 
doesn't make sense. It determines if the parameter is `ref` from 
the argument passed to the function at IFTI. You can't have IFTI 
if your function isn't a template.
I can only guess what you actually want and it might be what I 
proposed in one of the *DIP Ideas* forum, which I called 
`@universal ref`. It does not infer anything from the argument, 
but binds lvalues normally by reference and materializes rvalues 
at the caller side and passes them by reference as well.