libstdc++: Update LWG 4166 changes to concat_view::end() [PR120934]

In r15-4555-gf191c830154565 we proactively implemented the initial
proposed resolution for LWG 4166 which later turned out to be
insufficient, since we must also require equality_comparable of the
underlying iterators before concat_view could be a common range.

This patch implements the updated P/R, requiring all underlying
iterators to be forward (which implies equality_comparable) before
making concat_view common, which fixes the testcase from this PR.

	PR libstdc++/120934

libstdc++-v3/ChangeLog:

	* include/std/ranges (concat_view::end): Refine condition
	for returning an iterator instead of default_sentinel as
	per the updated P/R for LWG 4166.
	* testsuite/std/ranges/concat/1.cc (test05): New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/ranges/concat/1.cc b/libstdc++-v3/testsuite/std/ranges/concat/1.cc
index 1672191..f78ed08 100644
--- a/libstdc++-v3/testsuite/std/ranges/concat/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/concat/1.cc
@@ -99,6 +99,18 @@ test04()
   using type = decltype(v);
 }
 
+void
+test05()
+{
+  // PR libstdc++/120934 - views::concat is ill-formed depending on argument order
+  auto v1 = views::single(1);
+  std::vector<int> vec = {2, 3};
+  auto v2 = views::join(views::transform(vec, views::single));
+
+  static_assert( ranges::range<decltype(views::concat(v1, v2))> );
+  static_assert( ranges::range<decltype(views::concat(v2, v1))> );
+}
+
 int
 main()
 {
@@ -107,4 +119,5 @@ main()
   test02();
   test03();
   test04();
+  test05();
 }