libstdc++: Fix std::to_address for debug iterators (PR 93960)

It should be valid to use std::to_address on a past-the-end iterator,
but the debug mode iterators do a check for dereferenceable in their
operator->(). That check is generally useful, so rather than remove it
this changes std::__to_address to identify a debug mode iterator and
use base().operator->() to skip the check.

	PR libstdc++/93960
	* include/bits/ptr_traits.h (__to_address): Add special case for debug
	iterators, to avoid dereferenceable check.
	* testsuite/20_util/to_address/1_neg.cc: Adjust dg-error line number.
	* testsuite/20_util/to_address/debug.cc: New test.

4 files changed, 55 insertions, 2 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index e713fd235e9..c358977dfbb 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,11 @@
+2020-04-03  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/93960
+	* include/bits/ptr_traits.h (__to_address): Add special case for debug
+	iterators, to avoid dereferenceable check.
+	* testsuite/20_util/to_address/1_neg.cc: Adjust dg-error line number.
+	* testsuite/20_util/to_address/debug.cc: New test.
+
 2020-04-01  Andrea Corallo  <andrea.corallo@arm.com>
 
 	* testsuite/experimental/net/execution_context/use_service.cc:
diff --git a/libstdc++-v3/include/bits/ptr_traits.h b/libstdc++-v3/include/bits/ptr_traits.h
index 429f7fde520..8fd91bf7bf2 100644
--- a/libstdc++-v3/include/bits/ptr_traits.h
+++ b/libstdc++-v3/include/bits/ptr_traits.h
@@ -34,6 +34,10 @@
 
 #include <bits/move.h>
 
+#if __cplusplus > 201703L
+namespace __gnu_debug { struct _Safe_iterator_base; }
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -169,7 +173,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Ptr, typename... _None>
     constexpr auto
     __to_address(const _Ptr& __ptr, _None...) noexcept
-    { return std::__to_address(__ptr.operator->()); }
+    {
+      if constexpr (is_base_of_v<__gnu_debug::_Safe_iterator_base, _Ptr>)
+	return std::__to_address(__ptr.base().operator->());
+      else
+	return std::__to_address(__ptr.operator->());
+    }
 
   /**
    * @brief Obtain address referenced by a pointer to an object
diff --git a/libstdc++-v3/testsuite/20_util/to_address/1_neg.cc b/libstdc++-v3/testsuite/20_util/to_address/1_neg.cc
index ceb2141f10b..ee64a2281fb 100644
--- a/libstdc++-v3/testsuite/20_util/to_address/1_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/to_address/1_neg.cc
@@ -17,7 +17,7 @@
 
 // { dg-options "-std=gnu++2a" }
 // { dg-do compile { target c++2a } }
-// { dg-error "not a function pointer" "" { target *-*-* } 153 }
+// { dg-error "not a function pointer" "" { target *-*-* } 157 }
 
 #include <memory>
 
diff --git a/libstdc++-v3/testsuite/20_util/to_address/debug.cc b/libstdc++-v3/testsuite/20_util/to_address/debug.cc
new file mode 100644
index 00000000000..4555284416d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/to_address/debug.cc
@@ -0,0 +1,36 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <debug/vector>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  __gnu_debug::vector<int> v{1, 2, 3};
+  auto p = std::to_address(v.end());
+  VERIFY( p == v.data() + v.size() );
+}
+
+int
+main()
+{
+  test01();
+}