libstdc++: Fix std::allocator<void> for versioned namespace

Removing the allocator<void> specialization for the versioned namespace
breaks _Extptr_allocator<void> because the allocator<void>
specialization was still declared in <bits/memoryfwd.h>, making it an
incomplete type.  It wrong to remove that specialization anyway, because
it is still needed pre-C++20.

This removes the #if ! _GLIBCXX_INLINE_VERSION check, so that
allocator<void> is still explicitly specialized for the versioned
namespace, consistent with the normal unversioned namespace mode.

To make _Extptr_allocator<void> usable as a ProtoAllocator, this change
adds a default constructor and converting constructor. That is
consistent with std::allocator<void> since C++20 (and harmless to do for
earlier standards).

I'm also explicitly specializing allocator_traits<allocator<void>> so
that it doesn't need to use allocator<void>::construct and destroy.
Doing that allows those members to be removed, further simplifying
allocator<void>.  That new explicit specialization can delete the
allocate, deallocate and max_size members, which are always ill-formed
for allocator<void>.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/alloc_traits.h (allocator_traits): Add explicit
	specialization for allocator<void>. Improve doxygen comments.
	* include/bits/allocator.h (allocator<void>): Restore for the
	versioned namespace.
	(allocator<void>::construct, allocator<void>::destroy): Remove.
	* include/ext/extptr_allocator.h (_Extptr_allocator<void>):
	Add default constructor and converting constructor.

1 files changed, 104 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/alloc_traits.h b/libstdc++-v3/include/bits/alloc_traits.h
index 34412583064..05b584f742f 100644
--- a/libstdc++-v3/include/bits/alloc_traits.h
+++ b/libstdc++-v3/include/bits/alloc_traits.h
@@ -559,6 +559,110 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return __rhs; }
     };
 
+  /// Explicit specialization for std::allocator<void>.
+  template<>
+    struct allocator_traits<allocator<void>>
+    {
+      /// The allocator type
+      using allocator_type = allocator<void>;
+
+      /// The allocated type
+      using value_type = void;
+
+      /// The allocator's pointer type.
+      using pointer = void*;
+
+      /// The allocator's const pointer type.
+      using const_pointer = const void*;
+
+      /// The allocator's void pointer type.
+      using void_pointer = void*;
+
+      /// The allocator's const void pointer type.
+      using const_void_pointer = const void*;
+
+      /// The allocator's difference type
+      using difference_type = std::ptrdiff_t;
+
+      /// The allocator's size type
+      using size_type = std::size_t;
+
+      /// How the allocator is propagated on copy assignment
+      using propagate_on_container_copy_assignment = false_type;
+
+      /// How the allocator is propagated on move assignment
+      using propagate_on_container_move_assignment = true_type;
+
+      /// How the allocator is propagated on swap
+      using propagate_on_container_swap = false_type;
+
+      /// Whether all instances of the allocator type compare equal.
+      using is_always_equal = true_type;
+
+      template<typename _Up>
+	using rebind_alloc = allocator<_Up>;
+
+      template<typename _Up>
+	using rebind_traits = allocator_traits<allocator<_Up>>;
+
+      /// allocate is ill-formed for allocator<void>
+      static void*
+      allocate(allocator_type&, size_type, const void* = nullptr) = delete;
+
+      /// deallocate is ill-formed for allocator<void>
+      static void
+      deallocate(allocator_type&, void*, size_type) = delete;
+
+      /**
+       *  @brief  Construct an object of type `_Up`
+       *  @param  __a  An allocator.
+       *  @param  __p  Pointer to memory of suitable size and alignment for
+       *	       an object of type `_Up`.
+       *  @param  __args Constructor arguments.
+       *
+       *  Calls `__a.construct(__p, std::forward<_Args>(__args)...)`
+       *  in C++11, C++14 and C++17. Changed in C++20 to call
+       *  `std::construct_at(__p, std::forward<_Args>(__args)...)` instead.
+      */
+      template<typename _Up, typename... _Args>
+	static _GLIBCXX20_CONSTEXPR void
+	construct(allocator_type&, _Up* __p, _Args&&... __args)
+	noexcept(std::is_nothrow_constructible<_Up, _Args...>::value)
+	{
+#if __cplusplus <= 201703L
+	  ::new((void *)__p) _Up(std::forward<_Args>(__args)...);
+#else
+	  std::construct_at(__p, std::forward<_Args>(__args)...);
+#endif
+	}
+
+      /**
+       *  @brief  Destroy an object of type `_Up`
+       *  @param  __a  An allocator.
+       *  @param  __p  Pointer to the object to destroy
+       *
+       *  Invokes the destructor for `*__p`.
+      */
+      template<typename _Up>
+	static _GLIBCXX20_CONSTEXPR void
+	destroy(allocator_type&, _Up* __p)
+	noexcept(is_nothrow_destructible<_Up>::value)
+	{ std::_Destroy(__p); }
+
+      /// max_size is ill-formed for allocator<void>
+      static size_type
+      max_size(const allocator_type&) = delete;
+
+      /**
+       *  @brief  Obtain an allocator to use when copying a container.
+       *  @param  __rhs  An allocator.
+       *  @return `__rhs`
+      */
+      static _GLIBCXX20_CONSTEXPR allocator_type
+      select_on_container_copy_construction(const allocator_type& __rhs)
+      { return __rhs; }
+    };
+
 #if __cplusplus < 201703L
   template<typename _Alloc>
     inline void