[PATCH] libstdc++: Implement LWG 3820/3849 changes to cartesian_product_view

[Patrick Palka via Gcc-patches]

The LWG 3820 testcase revealed a bug in _M_advance, which this patch
also fixes.

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

        * include/std/ranges
        (cartesian_product_view::_Iterator::_Iterator): Remove
        constraint on default constructor as per LWG 3849.
        (cartesian_product_view::_Iterator::_M_prev): Adjust position
        of _Nm > 0 test as per LWG 3820.
        (cartesian_product_view::_Iterator::_M_advance): Perform bound
        checking only on sized cartesian products.
        * testsuite/std/ranges/cartesian_product/1.cc (test08): New test.
---
 libstdc++-v3/include/std/ranges               | 23 +++++++++++--------
 .../std/ranges/cartesian_product/1.cc         |  9 ++++++++
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 67566c6ebcf..14f38727198 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -8225,7 +8225,7 @@ namespace views::__adaptor
                                    range_reference_t<__maybe_const_t<_Const, 
_Vs>>...>;
     using difference_type = 
decltype(cartesian_product_view::_S_difference_type());
 
-    _Iterator() requires forward_range<__maybe_const_t<_Const, _First>> = 
default;
+    _Iterator() = default;
 
     constexpr
     _Iterator(_Iterator<!_Const> __i)
@@ -8390,12 +8390,12 @@ namespace views::__adaptor
     _M_prev()
     {
       auto& __it = std::get<_Nm>(_M_current);
-      if (__it == ranges::begin(std::get<_Nm>(_M_parent->_M_bases)))
-       {
-         __it = 
__detail::__cartesian_common_arg_end(std::get<_Nm>(_M_parent->_M_bases));
-         if constexpr (_Nm > 0)
+      if constexpr (_Nm > 0)
+       if (__it == ranges::begin(std::get<_Nm>(_M_parent->_M_bases)))
+         {
+           __it = 
__detail::__cartesian_common_arg_end(std::get<_Nm>(_M_parent->_M_bases));
            _M_prev<_Nm - 1>();
-       }
+         }
       --__it;
     }
 
@@ -8416,10 +8416,13 @@ namespace views::__adaptor
          if constexpr (_Nm == 0)
            {
 #ifdef _GLIBCXX_ASSERTIONS
-             auto __size = ranges::ssize(__r);
-             auto __begin = ranges::begin(__r);
-             auto __offset = __it - __begin;
-             __glibcxx_assert(__offset + __x >= 0 && __offset + __x <= __size);
+             if constexpr (sized_range<__maybe_const_t<_Const, _First>>)
+               {
+                 auto __size = ranges::ssize(__r);
+                 auto __begin = ranges::begin(__r);
+                 auto __offset = __it - __begin;
+                 __glibcxx_assert(__offset + __x >= 0 && __offset + __x <= 
__size);
+               }
 #endif
              __it += __x;
            }
diff --git a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc 
b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
index f52c2b96d58..56ff3d152c6 100644
--- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
@@ -201,6 +201,14 @@ test07()
   VERIFY( i == 5 );
 }
 
+void
+test08()
+{
+  // LWG 3820
+  auto r = std::views::cartesian_product(std::views::iota(0));
+  r.begin() += 3; // hard error
+}
+
 int
 main()
 {
@@ -211,4 +219,5 @@ main()
   test05();
   static_assert(test06());
   test07();
+  test08();
 }
-- 
2.40.0.rc0.57.g454dfcbddf