Unexpected foreach lowering

[Jonathan M Davis via Digitalmars-d-learn]

On Thursday, August 11, 2016 08:42:27 Steven Schveighoffer via Digitalmars-d-
learn wrote:
> On 8/11/16 12:28 AM, Jonathan M Davis via Digitalmars-d-learn wrote:
> > On Wednesday, August 10, 2016 21:00:01 Lodovico Giaretta via
> > Digitalmars-d-
> >
> > learn wrote:
> >> Wow. Thanks. I didn't know the compiler would try opSlice. I will
> >> file it.
> >
> > It does that so that you can use foreach with containers without having to
> > call something on the container. The idea is that the container will
> > implement opSlice and make it return a range over the container, and
> > foreach will then use that range to iterate over the container.
>
> I get that. But it shouldn't try opSlice *first* if the item itself is a
> range (and it does do this). Many random-access ranges define opSlice,
> and most of the time range[] returns this. But in this case, it doesn't.
>
> But it's a no-op for ranges, why waste time calling it?

I was just explaining what the deal with opSlice and containers was, since
the OP wasn't familiar with it. I wasn't really saying anything about the
suggested change. However, I have to agree that the suggested change is
likely a good one. As it stands, no range should be implementing opSlice
with no arguments, since it's a container function, not a range function. No
range trait tests for it, and no range-based code should be using it. It's
just supposed to be used on containers to get a range not on the ranges
themselves. But some folks put it on ranges anyway (IIRC, there are even at
least a couple of cases in Phobos where opSlice is implemented on ranges
when it shouldn't be), and it causes problems. There might be a reason why
it would be a bad idea to change it so that opSlice is ignored by foreach if
the type is a range, but I can't think of any right now. Unfortunately,
ranges and foreach have a tendency to get a bit funny thanks to the
differing behavior between types (e.g. ranges that are implicitly saved when
used with foreach vs those that aren't), so mucking around with it should be
done with care, but it at least seems like your suggestion to skip opSlice
if the range primitives are there is a good one.

- Jonathan M Davis