[libcxx-commits] [PATCH] D130295: [libc++] Uses operator<=> in string_view

[Nikolas Klauser via Phabricator via libcxx-commits]

philnik added inline comments.


================
Comment at: libcxx/include/string_view:803-810
+_LIBCPP_HIDE_FROM_ABI constexpr auto operator<=>(
+    basic_string_view<_CharT, _Traits> __lhs,
+    typename common_type<basic_string_view<_CharT, _Traits> >::type __rhs) noexcept {
+    if constexpr (requires { typename _Traits::comparison_category; })
+        return static_cast<typename _Traits::comparison_category >(__lhs.compare(__rhs) <=> 0);
+    else
+        return static_cast<weak_ordering>(__lhs.compare(__rhs) <=> 0);
----------------
What breaks if you drop the other two overloads? The compiler should automatically generate the `operator<=>(common_type_t<basic_string_view>, basic_string_view)` IIUC. Or is that just for `operator==`?


================
Comment at: libcxx/include/string_view:805
+    basic_string_view<_CharT, _Traits> __lhs,
+    typename common_type<basic_string_view<_CharT, _Traits> >::type __rhs) noexcept {
+    if constexpr (requires { typename _Traits::comparison_category; })
----------------
jloser wrote:
> Nit: can use `common_type_t` instead - here and elsewhere.
I think this should actually be `type_identity_t`. You could theoretically have something like
```
struct CharT {
  // all the stuff to make it char-like
};

struct CharTT {
  template <class Traits>
  CharTT(std::basic_string_view<CharT, Traits>);
 };

template <class Traits>
struct std::common_type<std::basic_string_view<CharT, Traits>, std::basic_string_view<CharT, Traits>> {
  using type = CharTT;
};
```
right? That would result in a hard error.



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  https://reviews.llvm.org/D130295/new/

https://reviews.llvm.org/D130295