[gcc r15-7835] libstdc++: Some concat_view bugfixes [PR115215, PR115218, LWG 4082]

[Patrick Palka]

https://gcc.gnu.org/g:81c6c99efa6a7afd3082785a9ab7fb64d2c93e1d

commit r15-7835-g81c6c99efa6a7afd3082785a9ab7fb64d2c93e1d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Mar 5 11:11:35 2025 -0500

    libstdc++: Some concat_view bugfixes [PR115215, PR115218, LWG 4082]
    
    - Use __builtin_unreachable to suppress a false-positive "control
      reaches end of non-void function" warning in the recursive lambda
      (which the existing tests failed to notice since test01 wasn't
      being called at runtime)
    - Relax the constraints on views::concat in the single-argument case
      as per PR115215
    - Add an input_range requirement to that same case as per LWG 4082
    - In the const-converting constructor of concat_view's iterator,
      don't require the first iterator to be default constructible
    
            PR libstdc++/115215
            PR libstdc++/115218
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges
            (concat_view::iterator::_S_invoke_with_runtime_index): Use
            __builtin_unreachable in recursive lambda to certify it always
            exits via 'return'.
            (concat_view::iterator::iterator): In the const-converting
            constructor, direct initialize _M_it.
            (views::_Concat::operator()): Adjust constraints in the
            single-argument case as per LWG 4082.
            * testsuite/std/ranges/concat/1.cc (test01): Call it at runtime
            too.
            (test04): New test.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/std/ranges               | 25 +++++++++++++------------
 libstdc++-v3/testsuite/std/ranges/concat/1.cc | 16 ++++++++++++++++
 2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index c0b1134ab328..def7527431cd 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9922,6 +9922,7 @@ namespace ranges
 	    return __f.template operator()<_Idx>();
 	  if constexpr (_Idx + 1 < sizeof...(_Vs))
 	    return __self.template operator()<_Idx + 1>();
+	  __builtin_unreachable();
 	}.template operator()<0>();
       }
 
@@ -9943,12 +9944,12 @@ namespace ranges
     constexpr
     _Iterator(_Iterator<!_Const> __it)
       requires _Const && (convertible_to<iterator_t<_Vs>, iterator_t<const _Vs>> && ...)
-    : _M_parent(__it._M_parent)
-    {
-      _M_invoke_with_runtime_index([this, &__it]<size_t _Idx>() {
-	_M_it.template emplace<_Idx>(std::get<_Idx>(std::move(__it._M_it)));
-      });
-    }
+    : _M_parent(__it._M_parent),
+      _M_it(_S_invoke_with_runtime_index([this, &__it]<size_t _Idx>() {
+	      return __base_iter(in_place_index<_Idx>,
+				 std::get<_Idx>(std::move(__it._M_it)));
+	    }, __it._M_it.index()))
+    { }
 
     constexpr decltype(auto)
     operator*() const
@@ -10186,12 +10187,12 @@ namespace ranges
 	requires __detail::__can_concat_view<_Ts...>
       constexpr auto
       operator() [[nodiscard]] (_Ts&&... __ts) const
-      {
-	if constexpr (sizeof...(_Ts) == 1)
-	  return views::all(std::forward<_Ts>(__ts)...);
-	else
-	  return concat_view(std::forward<_Ts>(__ts)...);
-      }
+      { return concat_view(std::forward<_Ts>(__ts)...); }
+
+      template<input_range _Range>
+      constexpr auto
+      operator() [[nodiscard]] (_Range&& __t) const
+      { return views::all(std::forward<_Range>(__t)); }
     };
 
     inline constexpr _Concat concat;
diff --git a/libstdc++-v3/testsuite/std/ranges/concat/1.cc b/libstdc++-v3/testsuite/std/ranges/concat/1.cc
index e5d10f476e93..16721912a37d 100644
--- a/libstdc++-v3/testsuite/std/ranges/concat/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/concat/1.cc
@@ -85,10 +85,26 @@ test03()
   VERIFY( ranges::equal(view2, std::vector{4, 5, 6, 1, 2, 3}) );
 }
 
+void
+test04()
+{
+  // PR libstdc++/115215 - views::concat rejects non-movable reference
+  int x[] = {1,2,3};
+  struct nomove {
+    nomove() = default;
+    nomove(const nomove&) = delete;
+  };
+  auto v = x | views::transform([](int) { return nomove{}; });
+  using type = decltype(views::concat(v));
+  using type = decltype(v);
+}
+
 int
 main()
 {
   static_assert(test01());
+  test01();
   test02();
   test03();
+  test04();
 }