libstdc++: Simplify std::aligned_storage and fix for versioned namespace [PR61458]

This simplifies the implementation of std::aligned_storage. For the
unstable ABI it also fixes the bug where its size is too large when the
default alignment is used. We can't fix that for the stable ABI though,
so just add a comment about the bug.

libstdc++-v3/ChangeLog:

	PR libstdc++/61458
	* doc/doxygen/user.cfg.in (GENERATE_BUGLIST): Set to NO.
	* include/std/type_traits (__aligned_storage_msa): Remove.
	(__aligned_storage_max_align_t): New struct.
	(__aligned_storage_default_alignment): New function.
	(aligned_storage): Use __aligned_storage_default_alignment for
	default alignment. Replace union with a struct containing an
	aligned buffer. Improve Doxygen comment.
	(aligned_storage_t): Use __aligned_storage_default_alignment for
	default alignment.

2 files changed, 60 insertions, 25 deletions

diff --git a/libstdc++-v3/doc/doxygen/user.cfg.in b/libstdc++-v3/doc/doxygen/user.cfg.in
index 8fe337a..ae50f6d 100644
--- a/libstdc++-v3/doc/doxygen/user.cfg.in
+++ b/libstdc++-v3/doc/doxygen/user.cfg.in
@@ -681,7 +681,7 @@ GENERATE_TESTLIST      = NO
 # list. This list is created by putting \bug commands in the documentation.
 # The default value is: YES.
 
-GENERATE_BUGLIST       = YES
+GENERATE_BUGLIST       = NO
 
 # The GENERATE_DEPRECATEDLIST tag can be used to enable (YES) or disable (NO)
 # the deprecated list. This list is created by putting \deprecated commands in
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 17ae2c4..c650094 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2244,39 +2244,74 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using add_pointer_t = typename add_pointer<_Tp>::type;
 #endif
 
-  template<std::size_t _Len>
-    struct __aligned_storage_msa
-    {
-      union __type
-      {
-	unsigned char __data[_Len];
-	struct __attribute__((__aligned__)) { } __align;
-      };
-    };
+  /// @cond undocumented
+
+  // Aligned to maximum fundamental alignment
+  struct __attribute__((__aligned__)) __aligned_storage_max_align_t
+  { };
+
+  constexpr size_t
+  __aligned_storage_default_alignment([[__maybe_unused__]] size_t __len)
+  {
+#if _GLIBCXX_INLINE_VERSION
+    using _Max_align
+      = integral_constant<size_t, alignof(__aligned_storage_max_align_t)>;
+
+    return __len > (_Max_align::value / 2)
+	     ? _Max_align::value
+# if _GLIBCXX_USE_BUILTIN_TRAIT(__builtin_clzg)
+	     : 1 << (__SIZE_WIDTH__ - __builtin_clzg(__len - 1u));
+# else
+	     : 1 << (__LLONG_WIDTH__ - __builtin_clzll(__len - 1ull));
+# endif
+#else
+    // Returning a fixed value is incorrect, but kept for ABI compatibility.
+    // XXX GLIBCXX_ABI Deprecated
+    return alignof(__aligned_storage_max_align_t);
+#endif
+  }
+  /// @endcond
 
   /**
-   *  @brief Alignment type.
+   *  @brief Aligned storage
+   *
+   *  The member typedef `type` is be a POD type suitable for use as
+   *  uninitialized storage for any object whose size is at most `_Len`
+   *  and whose alignment is a divisor of `_Align`.
+   *
+   *  It is important to use the nested `type` as uninitialized storage,
+   *  not the `std::aligned_storage` type itself which is an empty class
+   *  with 1-byte alignment. So this is correct:
+   *
+   *  `typename std::aligned_storage<sizeof(X), alignof(X)>::type m_xobj;`
+   *
+   *  This is wrong:
+   *
+   *  `std::aligned_storage<sizeof(X), alignof(X)> m_xobj;`
+   *
+   *  In C++14 and later `std::aligned_storage_t<sizeof(X), alignof(X)>`
+   *  can be used to refer to the `type` member typedef.
+   *
+   *  The default value of _Align is supposed to be the most stringent
+   *  fundamental alignment requirement for any C++ object type whose size
+   *  is no greater than `_Len` (see [basic.align] in the C++ standard).
    *
-   *  The value of _Align is a default-alignment which shall be the
-   *  most stringent alignment requirement for any C++ object type
-   *  whose size is no greater than _Len (3.9). The member typedef
-   *  type shall be a POD type suitable for use as uninitialized
-   *  storage for any object whose size is at most _Len and whose
-   *  alignment is a divisor of _Align.
+   *  @bug In this implementation the default value for _Align is always the
+   *  maximum fundamental alignment, i.e. `alignof(max_align_t)`, which is
+   *  incorrect. It should be an alignment value no greater than `_Len`.
    *
    *  @deprecated Deprecated in C++23. Uses can be replaced by an
-   *  array std::byte[_Len] declared with alignas(_Align).
+   *  array `std::byte[_Len]` declared with `alignas(_Align)`.
   */
-  template<std::size_t _Len, std::size_t _Align =
-	   __alignof__(typename __aligned_storage_msa<_Len>::__type)>
+  template<size_t _Len,
+	   size_t _Align = __aligned_storage_default_alignment(_Len)>
     struct
     _GLIBCXX23_DEPRECATED
     aligned_storage
     {
-      union type
+      struct type
       {
-	unsigned char __data[_Len];
-	struct __attribute__((__aligned__((_Align)))) { } __align;
+	alignas(_Align) unsigned char __data[_Len];
       };
     };
 
@@ -2776,8 +2811,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
   /// Alias template for aligned_storage
-  template<size_t _Len, size_t _Align =
-	    __alignof__(typename __aligned_storage_msa<_Len>::__type)>
+  template<size_t _Len,
+	   size_t _Align = __aligned_storage_default_alignment(_Len)>
     using aligned_storage_t _GLIBCXX23_DEPRECATED = typename aligned_storage<_Len, _Align>::type;
 
   template <size_t _Len, typename... _Types>