[committed] libstdc++: Fix resolution of LWG 4016 for std::ranges::to [PR112876]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

What I implemented in r14-6199-g45630fbcf7875b does not match what I
proposed for LWG 4016, and it imposes additional, unwanted requirements
on the emplace and insert member functions of the container being
populated.

libstdc++-v3/ChangeLog:

        PR libstdc++/112876
        * include/std/ranges (ranges::to): Do not try to use an iterator
        returned by the container's emplace or insert member functions.
        * testsuite/std/ranges/conv/1.cc (Cont4::emplace, Cont4::insert):
        Use the iterator parameter. Do not return an iterator.
---
 libstdc++-v3/include/std/ranges             | 10 +++-------
 libstdc++-v3/testsuite/std/ranges/conv/1.cc | 12 ++++++------
 2 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index fb9df3d3e79..be8475c0cb1 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9300,14 +9300,10 @@ namespace __detail
                    __c.emplace_back(*__it);
                  else if constexpr (requires { __c.push_back(*__it); })
                    __c.push_back(*__it);
+                 else if constexpr (requires { __c.emplace(__c.end(), *__it); 
})
+                   __c.emplace(__c.end(), *__it);
                  else
-                   {
-                     auto __end = __c.end();
-                     if constexpr (requires { __c.emplace(__end, *__it); })
-                       __end = __c.emplace(__end, *__it);
-                     else
-                       __end = __c.insert(__end, *__it);
-                   }
+                   __c.insert(__c.end(), *__it);
                  ++__it;
                }
              return __c;
diff --git a/libstdc++-v3/testsuite/std/ranges/conv/1.cc 
b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
index b5f861dedb3..6d6a708ab64 100644
--- a/libstdc++-v3/testsuite/std/ranges/conv/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
@@ -236,19 +236,19 @@ struct Cont4
   template<typename T>
     requires (Kind <= Emplace)
     && requires(C& c, T&& t) { c.emplace(c.end(), std::forward<T>(t)); }
-    typename C::iterator
-    emplace(typename C::iterator, T&& t)
+    void
+    emplace(typename C::iterator pos, T&& t)
     {
       kind = Emplace;
-      return c.emplace(c.end(), std::forward<T>(t));
+      c.emplace(pos, std::forward<T>(t));
     }
 
   template<typename T>
-    typename C::iterator
-    insert(typename C::iterator, T&& t)
+    void
+    insert(typename C::iterator pos, T&& t)
     {
       kind = Insert;
-      return c.insert(c.end(), std::forward<T>(t));
+      c.insert(pos, std::forward<T>(t));
     }
 
   // Required to satisfy reservable-container
-- 
2.43.0