[libcxx-commits] [PATCH] D102020: [libcxx][ranges] Add class ref_view.

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone added inline comments.


================
Comment at: libcxx/include/__ranges/ref_view.h:40-41
+
+    static void __test(_Range&);
+    static void __test(_Range&&) = delete;
+
----------------
ldionne wrote:
> tcanens wrote:
> > ldionne wrote:
> > > Those can be made private, and I would perhaps rename it to something like `__binds_to_lvalue_ref` or something like that.
> > > 
> > > Do you have a better understanding of what this function is used for than I do? IIUC, it's only used to check that `__tp` in the constructor below would bind to a lvalue, but not to a rvalue. I'm kind of wondering why the Standard is using this obtuse wording -- there must be a good reason?
> > This is the [LWG2993](https://cplusplus.github.io/LWG/issue2993) dance to avoid having an ICS from rvalues.
> Thanks for the context. I must say though, I was unable to construct a case where dropping `requires { __binds_to_lvalue_ref(declval<_Tp>()); }` would make a difference since we're already checking for `convertible_to<_Tp, _Range&>`.
> 
> In LWG2993, I understand why `reference_wrapper` used to be broken with the `reference_wrapper(T&&) = delete` overload, but I fail to see how this applies here. I guess it's not relevant to this review, I'm just curious in case you have time to elaborate.
I believe the only case this catches is when `T` is ambiguously convertible to both `R&` and `R&&`:
https://godbolt.org/z/hT4jraEo7

```
struct Evil {
    operator std::vector<int>& () const;
    operator std::vector<int>&& () const;
};
static_assert(!std::is_convertible_v<Evil, std::reference_wrapper<std::vector<int>>>);
static_assert(!std::is_convertible_v<Evil, std::ranges::ref_view<std::vector<int>>>);
```

I don't think there was ever a good reason for `reference_wrapper` to not-bind-to-rvalues, but it's true that `reference_wrapper` and `ref_view` are doing the exact same dance here and for exactly isomorphic reasons. (And that's a good reason to keep the naming convention the same, btw. `reference_wrapper` calls the helper function `__fun`, and I think we should do exactly the same here.)


Repository:
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  https://reviews.llvm.org/D102020/new/

https://reviews.llvm.org/D102020