[gcc r16-8868] libstdc++: Make std::unique_ptr<void>::operator* SFINAE-friendly

[Jonathan Wakely]

https://gcc.gnu.org/g:2a0f7d010e38e06c479ab37caf012509eae8e7d2

commit r16-8868-g2a0f7d010e38e06c479ab37caf012509eae8e7d2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 30 19:39:44 2026 +0100

    libstdc++: Make std::unique_ptr<void>::operator* SFINAE-friendly
    
    This implements LWG 4324, "unique_ptr<void>::operator* is not
    SFINAE-friendly", approved in Croydon, 2026.
    
    The noexcept-specifier added to C++23 by LWG 2762 is ill-formed if the
    pointer type cannot be dereferenced, which means that code which was
    checking whether the function exists (e.g. in a SFINAE context) no
    longer works. Such code was always questionable, because the function
    body was ill-formed if the pointer isn't dereferenceable, so the SFINAE
    check was probably giving the wrong answer, but it was possible to ask
    the question. Since LWG 2762 just asking the question can produce an
    error outside the immediate context, so operator* is no longer
    SFINAE-friendly.
    
    LWG 4324 adds a constraint to the function, so that it doesn't
    participate in overload resolution if it would be ill-formed. That's
    easy to implement for C++20 because we can just add a requires-clause.
    
    For C++11/14/17 we can't constrain it easily, so just adjust the
    noexcept-specifier so that it's not ill-formed. This still means you get
    the wrong answer (i.e. it looks like unique_ptr<void>::operator* is
    callable) but there's no error outside the immediate context. This
    restores the original semantics before the LWG 2762 change, for better
    or worse.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/unique_ptr.h (unique_ptr::_Nothrow_deref): New
            helper for pre-C++20.
            (unique_ptr::operator*): Either constrain or use _Nothrow_deref.
            * testsuite/20_util/unique_ptr/lwg4324.cc: New test.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    (cherry picked from commit 72fc51b1ac6451b2986d3351158b46b36827caf6)

Diff:
---
 libstdc++-v3/include/bits/unique_ptr.h             | 21 ++++++++++++++++++++-
 .../testsuite/20_util/unique_ptr/lwg4324.cc        | 22 ++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h
index 5b7c8938ff74..f4b2e33d13de 100644
--- a/libstdc++-v3/include/bits/unique_ptr.h
+++ b/libstdc++-v3/include/bits/unique_ptr.h
@@ -289,6 +289,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __not_<is_array<_Up>>
         >;
 
+#if ! __cpp_concepts
+      template<typename _Ptr, typename = void>
+	struct _Nothrow_deref
+	: false_type { };
+
+      template<typename _Ptr>
+	struct _Nothrow_deref<_Ptr, __void_t<decltype(*std::declval<_Ptr>())>>
+	: __bool_constant<noexcept(*std::declval<_Ptr>())> { };
+#endif
+
     public:
       // Constructors.
 
@@ -442,7 +452,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       /// Dereference the stored pointer.
       _GLIBCXX23_CONSTEXPR
       typename add_lvalue_reference<element_type>::type
-      operator*() const noexcept(noexcept(*std::declval<pointer>()))
+      operator*() const
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2762. unique_ptr operator*() should be noexcept
+      // 4324. unique_ptr<void>::operator* is not SFINAE-friendly
+#if __cpp_concepts
+      noexcept(noexcept(*std::declval<pointer>()))
+      requires requires { *std::declval<pointer>(); }
+#else
+      noexcept(_Nothrow_deref<pointer>::value)
+#endif
       {
 #if _GLIBCXX_USE_BUILTIN_TRAIT(__reference_converts_from_temporary)
 	// _GLIBCXX_RESOLVE_LIB_DEFECTS
diff --git a/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc
new file mode 100644
index 000000000000..13ebfb929fca
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++11 } }
+
+// LWG 4324. unique_ptr<void>::operator* is not SFINAE-friendly
+
+#include <memory>
+
+template<class T, bool = true>
+struct nothrow_dereference
+: std::false_type { };
+
+template<class T>
+struct nothrow_dereference<T, noexcept(*std::declval<T>())>
+: std::true_type { };
+
+static_assert( nothrow_dereference<std::unique_ptr<int>>::value, "");
+static_assert( ! nothrow_dereference<std::unique_ptr<void>>::value, "");
+
+#if __cplusplus >= 202002L // C++20
+template<class T> concept dereferenceable = requires(T& t) { *t; };
+static_assert( dereferenceable<std::unique_ptr<int>>);
+static_assert(!dereferenceable<std::unique_ptr<void>>);
+#endif