byte + byte = int: why?

[jfondren]

On Sunday, 29 August 2021 at 15:57:18 UTC, Paul Backus wrote:
> On Sunday, 29 August 2021 at 15:42:18 UTC, Ali Çehreli wrote:
>> Depending on the situation, you may want to use std.conv.to, 
>> which does a value range check and throws an exception to 
>> prevent an error:
>>
>>     byte foo(byte a, byte b) {
>>       import std.conv : to;
>>       return (a + b).to!byte;
>>     }
>>
>>     void main() {
>>       foo(42, 42);    // Works
>>       foo(100, 100);  // Throws ConvOverflowException
>>     }
>
> I was going to suggest `std.experimental.checkedint` as an 
> alternative here, but it turns out that it does integer 
> promotion too--`Checked!byte + Checked!byte == Checked!int`.
>
> This seems obviously wrong to me, but according to run.dlang.io 
> it's always worked that way.

(a.checked + b).get is int, but this works:

```d
import std.experimental.checkedint;

byte foo(byte a, byte b) {
     auto c = a.checked;
     c += b;
     return c.get;
}

unittest {
     import std.exception : assertThrown;
     import core.exception : AssertError;

     foo(42, 42);
     assertThrown!AssertError(foo(100, 100));
}
```

... after Phobos is patched.

```
error: undefined identifier ‘Lhs’, did you mean alias ‘Rhs’?
```