difficulties with const structs and alias this / template functions

[Dennis]

On Monday, 19 November 2018 at 01:13:29 UTC, Stanislav Blinov 
wrote:
> You just dismissed that second to last sentence, did you? :)

I don't know what you mean with it. It's not that I'm trying to 
be sneaky or lazy really trying to modify the const parameter 
when I should treat it properly. And as the author I'm fine with 
Unqualing everything if that's needed, my concern is when another 
person using my type tries to write his own function:

```
q16 pow(q16 base, int exponent) {
   q16 result = 1;
   foreach(i; 0..exponent) result *= base;
   return result;
}

const q16 x = 3;
writeln(pow(x, 3)); //works!
```

He then wants to make it more generic, so he rewrites:

```
Q pow(Q base, int exponent) if (isFixedPoint!Q) {
   Q result = 1;
   foreach(i; 0..exponent) result *= base;
   return result;
}
```

And initially it seems to work, but as soon as it is used with 
const it breaks as `result` can't be mutated anymore. I'd like to 
set the example of writing proper generic functions, and if there 
is something simpler than importing Unqual I'd prefer that over 
my current solution. If there isn't, I'll just need to write a 
"don't forget to Unqual const" comment.

> ```
> T f0(T)(inout T x, inout T y) { return x + y; }
> ```
>
> ;)

What does inout do here? If the return type is also inout(T) I 
know that the return type gets the same qualifiers as inout 
parameters. But in this example, the return value is still T.

> I know it's a bit painful though. In fact, Phobos also suffers 
> from it. In std.numeric:

Yuck!