philnik added a comment.
In D151625#4380735 <https://reviews.llvm.org/D151625#4380735>, @erichkeane
wrote:
> I'm a little dense today perhaps, but I don't get the use case for this? Can
> you ELI-EWG? Is this an attribute we expect our users to use? The name is
> horrifyingly long for users to use, but perhaps that is a good thing.
This is useful for libraries which have ADL requirements. Something like
template <class T>
struct Holder {
T v;
};
struct Incomplete;
template <class T>
class TupleImpl {
T v;
bool operator==(const TupleImpl&) const = default;
};
template <class T>
class Tuple : public TupleImpl<T> {
bool operator==(const Tuple&) const = default; // ADL call here
};
void func() {
Tuple<Holder<Incomplete>*> f; // Instantiation fails because Incomplete
isn't defined
}
has to work for `std::tuple` etc. to make them
`__is_trivially_equality_comparable`, but the defaulted `operator==` will
always try ADL (at least that's my understanding). To not make ADL calls, this
attribute can be used instead, which basically say "trust me, my operator==
does what the defaulted one would do".
Not sure what you mean by "Can you ELI-EWG?".
I agree that the name isn't great, but I couldn't come up with anything better
that is still descriptive, and this attribute will probably only be used by
very few libraries anyways (maybe just libc++, since this is pure QoI). As I
said in the attribute description, the defaulted operator should be used if
possible (which is probably the case in 99.9% of use cases).
(Note to self: maybe add the example as a test)
Repository:
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