[committed] libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

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.
---
 libstdc++-v3/include/std/array                |  4 ++--
 .../23_containers/array/creation/111512.cc    | 24 +++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100644 
libstdc++-v3/testsuite/23_containers/array/creation/111512.cc

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));
+}
-- 
2.41.0