[PATCH 3/3, 11 backport] libstdc++: invalid default init in _CachedPosition [PR101231]

[Patrick Palka <ppalka@redhat.com>]

The primary template for _CachedPosition is a dummy implementation for
non-forward ranges, the iterators for which generally can't be cached.
Because this implementation doesn't actually cache anything, _M_has_value
is defined to be false and so calls to _M_get (which are always guarded
by _M_has_value) are unreachable.

Still, to suppress a "control reaches end of non-void function" warning
I made _M_get return {}, but after P2325 input iterators are no longer
necessarily default constructible so this workaround now breaks valid
programs.

This patch fixes this by instead using __builtin_unreachable to squelch
the warning.

	PR libstdc++/103904
	PR libstdc++/101231

libstdc++-v3/ChangeLog:

	* include/std/ranges (_CachedPosition::_M_get): For non-forward
	ranges, just call __builtin_unreachable.
	* testsuite/std/ranges/istream_view.cc (test05): New test.

(cherry picked from commit 1af937eb6246ad7f63ebff03590e9eede33aca81)
---
 libstdc++-v3/include/std/ranges                   |  2 +-
 libstdc++-v3/testsuite/std/ranges/istream_view.cc | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index bf6cfae2a6e..a4228ba9aa0 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1221,7 +1221,7 @@ namespace views::__adaptor
 	_M_get(const _Range&) const
 	{
 	  __glibcxx_assert(false);
-	  return {};
+	  __builtin_unreachable();
 	}
 
 	constexpr void
diff --git a/libstdc++-v3/testsuite/std/ranges/istream_view.cc b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
index af76a1ab39e..f5c0c2a6bb0 100644
--- a/libstdc++-v3/testsuite/std/ranges/istream_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
@@ -83,6 +83,17 @@ test04()
   static_assert(!std::forward_iterator<It>);
 }
 
+void
+test05()
+{
+  // PR libstdc++/101231
+  auto words = std::istringstream{"42"};
+  auto is = ranges::istream_view<int>(words);
+  auto r = is | views::filter([](auto) { return true; });
+  for (auto x : r)
+    ;
+}
+
 void
 test06()
 {
@@ -99,5 +110,6 @@ main()
   test02();
   test03();
   test04();
+  test05();
   test06();
 }
-- 
2.35.1.102.g2b9c120970