[gcc r14-10708] libstdc++: Avoid forming T* in unique_ptr(auto_ptr<U>&&) constraints [PR116529]

[Jonathan Wakely]

https://gcc.gnu.org/g:4d88724c2d804e126d63aed77fa8c2c333e99396

commit r14-10708-g4d88724c2d804e126d63aed77fa8c2c333e99396
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 29 13:47:15 2024 +0100

    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.
    
    (cherry picked from commit a001d515059ba4647169f8c17967d08bbe41cb7a)

Diff:
---
 libstdc++-v3/include/bits/unique_ptr.h             |  5 ++--
 .../20_util/unique_ptr/creation/116529.cc          | 35 ++++++++++++++++++++++
 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 0f600db32f94..edcff78bff9f 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 000000000000..323fc7cb27ce
--- /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();
+}