[gcc r14-4252] libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

[Jonathan Wakely]

https://gcc.gnu.org/g:77cf3773021b0a20d89623e09d620747a05588ec

commit r14-4252-g77cf3773021b0a20d89623e09d620747a05588ec
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 21 09:14:57 2023 +0100

    libstdc++: Prevent unwanted ADL in std::to_array [PR111512]
    
    As noted in PR c++/111512, GCC does ADL for __builtin_memcpy if it is
    unqualified, which can cause errors for template argument types which
    cannot be completed.
    
    Casting the memcpy arguments to void* prevents ADL from considering the
    problem type.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/111511
            PR c++/111512
            * include/std/array (to_array): Cast memcpy arguments to void*.
            * testsuite/23_containers/array/creation/111512.cc: New test.

Diff:
---
 libstdc++-v3/include/std/array                     |  4 ++--
 .../23_containers/array/creation/111512.cc         | 24 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index 0e32d7b52d0..c4d534c3a34 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -432,7 +432,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    {
 	      array<remove_cv_t<_Tp>, _Nm> __arr;
 	      if (!__is_constant_evaluated() && _Nm != 0)
-		__builtin_memcpy(__arr.data(), __a, sizeof(__a));
+		__builtin_memcpy((void*)__arr.data(), (void*)__a, sizeof(__a));
 	      else
 		for (size_t __i = 0; __i < _Nm; ++__i)
 		  __arr._M_elems[__i] = __a[__i];
@@ -461,7 +461,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    {
 	      array<remove_cv_t<_Tp>, _Nm> __arr;
 	      if (!__is_constant_evaluated() && _Nm != 0)
-		__builtin_memcpy(__arr.data(), __a, sizeof(__a));
+		__builtin_memcpy((void*)__arr.data(), (void*)__a, sizeof(__a));
 	      else
 		for (size_t __i = 0; __i < _Nm; ++__i)
 		  __arr._M_elems[__i] = __a[__i];
diff --git a/libstdc++-v3/testsuite/23_containers/array/creation/111512.cc b/libstdc++-v3/testsuite/23_containers/array/creation/111512.cc
new file mode 100644
index 00000000000..b0f25a62153
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/array/creation/111512.cc
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++20 } }
+
+// Bug libstdc++/111511 - Incorrect ADL in std::to_array in GCC 11/12/13
+// Bug c++/111512 - GCC's __builtin_memcpy can trigger ADL
+
+#include <array>
+#include <utility>
+
+struct incomplete;
+
+template<class T>
+struct holder {
+    T t; // { dg-bogus "'holder<T>::t' has incomplete type" }
+};
+
+// A complete type that cannot be used as an associated type for ADL.
+using adl_bomb = holder<incomplete>*;
+
+int main()
+{
+    adl_bomb a[1]{};
+    (void) std::to_array(a);
+    (void) std::to_array(std::move(a));
+}