opIndex negative index?

[tsbockman]

On Thursday, 21 January 2021 at 14:00:28 UTC, cerjones wrote:
> I'm a bit unsure if this is reasonable or not.
>
> Thoughts?

Signed indexes are fine, if there is a principled reason to use 
them. Just make sure that you are consistent by making `length`, 
`opDollar`, etc. use signed types, as well.

Otherwise, you're laying a trap for users of the API who may 
accidentally write code with mixed signed-unsigned integer 
operations. That's a big problem, because mixed signed-unsigned 
integer operations are done incorrectly by default, often with no 
warnings, in D and some other C-like languages.

Consistently using signed types for lengths and indexes will make 
the API incompatible with some Phobos (D's `std`) range code, but 
you should get a compile-time error message. Mixing signed and 
unsigned, on the other hand, is likely to result in subtly wrong 
code that compiles without error, but sometimes does the wrong 
thing.