libstdc++: Avoid forming T* in unique_ptr(auto_ptr<U>&&) constraints [PR116529]

PR 116529 shows that std::unique_ptr<X&, D> is currently unusable
because the constructor taking std::auto_ptr (which is a non-standard
extension since C++17) tries to form the invalid type X&* during
overload resolution. We can use the `pointer` type in the constructor
constraints, instead of trying to form an invalid type. The
std::auto_ptr constructor can never actually match for the case where
element_type is a reference, so we just need it to produce a
substitution failure instead of being ill-formed.

LWG 4144 might make std::unique_ptr<X&, D> ill-formed, which would
invalidate this new test. We would have to remove this test in that
case. Using `pointer` in the constructor from std::auto_ptr would not be
needed to support the std::unique_ptr<X&, D> case, but would not cause
any harm either.

libstdc++-v3/ChangeLog:

	PR libstdc++/116529
	* include/bits/unique_ptr.h (unique_ptr(auto_ptr<U>&&)):
	Use pointer instead of T*.
	* testsuite/20_util/unique_ptr/creation/116529.cc: New test.

2 files changed, 38 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index 0f600db..edcff78 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -379,8 +379,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
       /// Converting constructor from @c auto_ptr
-      template<typename _Up, typename = _Require<
-	       is_convertible<_Up*, _Tp*>, is_same<_Dp, default_delete<_Tp>>>>
+      template<typename _Up,
+	       typename = _Require<is_convertible<_Up*, pointer>,
+				   is_same<_Dp, default_delete<_Tp>>>>
 	unique_ptr(auto_ptr<_Up>&& __u) noexcept;
 #pragma GCC diagnostic pop
 #endif
diff --git a/libstdc++-v3/testsuite/20_util/unique_ptr/creation/116529.cc b/libstdc++-v3/testsuite/20_util/unique_ptr/creation/116529.cc
new file mode 100644
index 0000000..323fc7c
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/unique_ptr/creation/116529.cc
@@ -0,0 +1,35 @@
+// { dg-do run { target c++11 } }
+
+// Bug libstdc++/116529 - Construction of unique_ptr with reference type
+// is rejected because of auto_ptr constructor
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+int count = 0;
+
+struct X
+{
+  ~X() { ++count; }
+};
+
+struct deleter : std::default_delete<X>
+{
+  using pointer = X*;
+};
+
+void
+test01()
+{
+  {
+    std::unique_ptr<X&, deleter> up(new X);
+    // { dg-bogus "forming pointer to reference" "" { target *-*-* } 0 }
+    VERIFY( count == 0 );
+  }
+  VERIFY( count == 1 );
+}
+
+int main()
+{
+  test01();
+}