[libc++][ranges] LWG3505: split_view::outer-iterator::operator++ misspecified (#155160)

Implemented in LLVM15:
e53c461

This LWG concerns`lazy_split_view` despite the outdated title.

Closes #104320

# References

- https://wg21.link/LWG3505
- https://wg21.link/range.lazy.split.outer

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/docs/Status/Cxx23Issues.csv

@@ -58,7 +58,7 @@
 "`LWG3495 <https://wg21.link/LWG3495>`__","``constexpr launder`` makes pointers to inactive members of unions usable","2021-02 (Virtual)","|Nothing To Do|","","`#104316 <https://github.com/llvm/llvm-project/issues/104316>`__",""
 "`LWG3500 <https://wg21.link/LWG3500>`__","``join_view::iterator::operator->()`` is bogus","2021-02 (Virtual)","|Complete|","14","`#104318 <https://github.com/llvm/llvm-project/issues/104318>`__",""
 "`LWG3502 <https://wg21.link/LWG3502>`__","``elements_view`` should not be allowed to return dangling reference","2021-02 (Virtual)","|Complete|","16","`#104319 <https://github.com/llvm/llvm-project/issues/104319>`__",""
-"`LWG3505 <https://wg21.link/LWG3505>`__","``split_view::outer-iterator::operator++`` misspecified","2021-02 (Virtual)","","","`#104320 <https://github.com/llvm/llvm-project/issues/104320>`__",""
+"`LWG3505 <https://wg21.link/LWG3505>`__","``split_view::outer-iterator::operator++`` misspecified","2021-02 (Virtual)","|Complete|","15","`#104320 <https://github.com/llvm/llvm-project/issues/104320>`__",""
 "","","","","","",""
 "`LWG2774 <https://wg21.link/LWG2774>`__","``std::function`` construction vs assignment","2021-06 (Virtual)","","","`#104321 <https://github.com/llvm/llvm-project/issues/104321>`__",""
 "`LWG2818 <https://wg21.link/LWG2818>`__","``::std::`` everywhere rule needs tweaking","2021-06 (Virtual)","|Nothing To Do|","","`#104322 <https://github.com/llvm/llvm-project/issues/104322>`__",""

libcxx/test/std/ranges/range.adaptors/range.lazy.split/range.lazy.split.outer/increment.pass.cpp

@@ -75,6 +75,56 @@ constexpr bool test() {
     }
   }
 
+  // LWG3505
+  {
+    using namespace std::string_view_literals;
+
+    { // Motivational example
+      auto v = std::views::lazy_split("xxyx"sv, "xy"sv);
+
+      {
+        auto i = v.begin();
+        assert(std::ranges::equal(*i, "x"s));
+
+        decltype(auto) i2 = ++i;
+        static_assert(std::is_lvalue_reference_v<decltype(i2)>);
+        assert(std::ranges::equal(*i2, "x"s));
+      }
+
+      {
+        auto i = v.begin();
+        assert(std::ranges::equal(*i, "x"s));
+
+        decltype(auto) i2 = i++;
+        static_assert(!std::is_reference_v<decltype(i2)>);
+        assert(std::ranges::equal(*i2, "x"s));
+        assert(std::ranges::equal(*i, "x"s));
+      }
+    }
+    {
+      auto v = std::views::lazy_split("zzht"sv, "zh"sv);
+
+      {
+        auto i = v.begin();
+        assert(std::ranges::equal(*i, "z"s));
+
+        decltype(auto) i2 = ++i;
+        static_assert(std::is_lvalue_reference_v<decltype(i2)>);
+        assert(std::ranges::equal(*i2, "t"s));
+      }
+
+      {
+        auto i = v.begin();
+        assert(std::ranges::equal(*i, "z"s));
+
+        decltype(auto) i2 = i++;
+        static_assert(!std::is_reference_v<decltype(i2)>);
+        assert(std::ranges::equal(*i2, "z"s));
+        assert(std::ranges::equal(*i, "t"s));
+      }
+    }
+  }
+
   return true;
 }