VRP and signed <-> unsigned conversion

Steven Schveighoffer schveiguy at gmail.com
Wed Dec 15 18:43:38 UTC 2021

On 12/15/21 1:04 PM, Quirin Schroll wrote:
> On Wednesday, 15 December 2021 at 14:39:09 UTC, Steven Schveighoffer wrote:
>> […]
>> I'm wondering:
>> 1. Does it make sense for this to be valid? Should we reexamine 
>> unsigned <-> signed implicit casting?
> I don't understand why `byte` should be implicitly convertible to 
> `ubyte`. Seeing this as is, I think this is a bug.

If it's a bug, it's a bug in design. From C, that D inherited.

>> 2. If the above rewrite is possible, shouldn't VRP just allow this 
>> conversion? i.e. a type that has an unsigned/signed counterpart should 
>> be assignable if the signed/unsigned can accept the range.
> To me, it seems that VRP is designed around mathematical intuition in 
> which the integer types are seen as {−2ⁿ⁻¹, …, 2ⁿ⁻¹−1} and {0, …, 2ⁿ−1} 
> for *n* appropriate. (I hope the Unicode superscripts render properly.)

(they did)

> The problem starts with subtraction. If *x, y* ∈ {0, …, 15}, then *x* − 
> *y* ∈ {−15, …, 15}. A lot of professional people know that (at least the 
> unsigned types) implement arithmetic modulo 2ⁿ, so *x* − *y* is 
> well-defined. However, you *can* see unsigned types as positive types 
> (e.g. when taking the length of an array); in this case, subtraction *x* 
> − *y* makes no sense when *y* > *x*.
> I guess the whole problem comes from the double-role unsigned types 
> play: positive numbers vs. mod-2ⁿ numbers; a triple-role with 
> bit-operations.

The impetus of VRP was simple -- C allows implicit casting between *all* 
integer types. D did not want to do that due to the errors it causes. 
However, since integer promotion is mimicked from C, all math is done at 
integer level (this makes C code that's compiled with D code behave 
similarly). However, it's quite painful to have to cast everything with 
integers that are smaller than 32-bit ints, so VRP is used to assist in 
making sure you don't have to cast if the compiler can prove the value fits.

But this quirk of allowing implicit casting between unsigned and signed 
violates that spirit. To me it feels like a bad code smell. I also have 
issues with the character types being considered as integers. D could do 
a lot better in this area I think.


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