[PATCH] D151625: [clang] Add `clang::equality_operator_compares_members_lexicographically`

[Erich Keane via Phabricator via cfe-commits]

erichkeane added a comment.

In D151625#4382227 <https://reviews.llvm.org/D151625#4382227>, @philnik wrote:

> 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)

ELI-EWG: "Explain-like-I'm Evolution Working Group" :)  Its a touch of a joke, there is a common reddit 'ELI5' (Explain like I'm 5), so I'm equating the C++ EWG to needing 5-year-old-esque explanations.

I can see the value to that I guess?  I don't have a good idea on the name, but perhaps folks from @clang-language-wg might have an idea?  Either way, I want to give it a little time for folks to comment.

That said, it DOES seem that the pre-commit-CI failures are related, so those need fixing as well.


Repository:
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  https://reviews.llvm.org/D151625/new/

https://reviews.llvm.org/D151625