Make -preview=intpromote not suck

[Steven Schveighoffer]

On 10/27/20 7:13 PM, Dukc wrote:
> On Monday, 26 October 2020 at 13:06:09 UTC, Steven Schveighoffer wrote:
>> [snip]
>>
> 
> Wouldn't it be simpler if integer promoting expressions were implicitly 
> castable to types they were promoted from, meaning:
> 
> ```
> short a = 1, b = 2;
> short c = a+b; //fine, int promoted from short implicitly castable back
> auto i = a+b; //int, because the type is still int if not casted
> short d = cast(short)i; //cast required here, promotion reversibility 
> applies only for expressions
> byte e = cast(byte)(a+b); //cast required because byte is smaller than 
> short
> ```
> 
> Won't solve both your problem and a whole host of others with `byte`s 
> and `short`s?
> 
> It should not be too hard to implement either, as array literals already 
> do the same. They are implicitly castable to static arrays of their own 
> size, while their type (dynamic array) is not.

I don't know if this is OK or correct. For example

short c = a * b; => lots of truncation gonna happen.

Note that C allows this, and D does not, on purpose.

I wanted to focus on the problem that *currently* compiles, will *not* 
compile after the change, and *behaves exactly the same* even with 
mitigation (casting). In other words, we would require lots of mindless 
busywork (and/or break lots of old code) for zero benefit.

Essentially, I like the idea of the intpromote preview. But this one 
problem is going to derail its adoption.

-Steve