The point of const, scope, and other attributes

[Joseph Rushton Wakeling]

On Saturday, 14 May 2022 at 01:11:03 UTC, Ali Çehreli wrote:
> So, I can't imagine a use case where lvalue mutation is 
> important but rvalue mutation is not.

Maybe for a function that wants to define optional out parameters 
that the caller can care about or not?  It's a trivial example, 
but something like:

```D
import std.stdio: writefln;

void foo () (int input,
              auto ref int optional_output = 0 /* dummy rvalue 
default */)
{
     writefln!"Input received: %s"(input);
     optional_output = input * 2;
}

void main()
{
     foo(7);  // `optional_out` parameter is effectively ignored
     writefln("Output parameter was not used");
     int output;
     foo(19, output);  // `optional_out` parameter writes to lvalue
     writefln!"Output received: %s"(output);
     assert(output == 38);
}
```

A similar pattern might also be useful for passing optional 
context object to which a function might write, say, information 
on its progress.

Not sure that either of these would be wise, but at least they 
are conceivable options.