aaron.ballman added a reviewer: erichkeane.
aaron.ballman added a comment.
In D129835#4287866 <https://reviews.llvm.org/D129835#4287866>, @cjdb wrote:
>>> With [[discardable]] one just needs to push/pop at the extremes of a file
>>> (and if a future version of module maps supports global pragmas for a
>>> module, there too, but that's a discussion that requires a design doc).
>>
>> I understood that, I just don't think that's a good thing. This is basically
>> an attribute that says "I know we said we wanted everything here to be
>> nodiscard, but JUST KIDDING not this one!" which is not a very clean
>> approach to writing headers.
>
> @aaron.ballman Ugh, I've finally come up with a good use-case for
> `[[discardable]]`.
>
> class [[nodiscard]] iterator {
> public:
> // usual iterator stuff
>
> [[clang::discardable]] iterator operator++(int);
> [[clang::discardable]] iterator operator--(int);
> };
>
> I hate it, but the alternative is to decorate everything else with
> `[[nodiscard]]` just to facilitate these two operators. It's a compelling
> use-case, but I'm not sure if it's compelling enough on its own. WDYT?
> (I personally think that those two should be nodiscard, but `i++` is
> pervasive enough that it might never be practical to correct.)
I'm still not super motivated because I think this encourages a confused
design, because marking a *type* as `nodiscard` is making a promise about the
type as a whole. That asserts there's *never* a valid time to ignore a value of
the type. If you have times when that type is reasonable to ignore, then the
type isn't actually non-discardable, and you should mark the methods returning
the type. (IOW, "nodiscard" is part of the contract for the type in some
limited ways even if it's not reflected by the type system.)
What's more, I think `iterator` is a case where it'd be a mistake to mark the
type `nodiscard`, because there are plenty times when it's valid to ignore the
type (consider many of the functions in `<algorithm>`, like `std::copy()`). I
think it's more appropriate to mark the functions returning an iterator rather
than the type itself.
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