[default.allocator] - C++17 → C++20

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 ### The default allocator <a id="default.allocator">[[default.allocator]]</a>
-All specializations of the default allocator satisfy the allocator
-completeness requirements ([[allocator.requirements.completeness]]).
 ``` cpp
 namespace std {
   template<class T> class allocator {
    public:
     using value_type                             = T;
     using propagate_on_container_move_assignment = true_type;
     using is_always_equal                        = true_type;
-    allocator() noexcept;
-    allocator(const allocator&) noexcept;
-    template <class U> allocator(const allocator<U>&) noexcept;
-    ~allocator();
-    T* allocate(size_t n);
-    void deallocate(T* p, size_t n);
   };
 }
 ```
-#### `allocator` members <a id="allocator.members">[[allocator.members]]</a>
 Except for the destructor, member functions of the default allocator
-shall not introduce data races ([[intro.multithread]]) as a result of
 concurrent calls to those member functions from different threads. Calls
 to these functions that allocate or deallocate a particular unit of
 storage shall occur in a single total order, and each such deallocation
 call shall happen before the next allocation (if any) in this order.
 ``` cpp
-T* allocate(size_t n);
 ```
-*Returns:* A pointer to the initial element of an array of storage of
-size `n` `* sizeof(T)`, aligned appropriately for objects of type `T`.
-*Remarks:* the storage is obtained by calling
-`::operator new` ([[new.delete]]), but it is unspecified when or how
-often this function is called.
-*Throws:* `bad_alloc` if the storage cannot be obtained.
 ``` cpp
-void deallocate(T* p, size_t n);
 ```
-*Requires:* `p` shall be a pointer value obtained from `allocate()`. `n`
-shall equal the value passed as the first argument to the invocation of
 allocate which returned `p`.
 *Effects:* Deallocates the storage referenced by `p` .
-*Remarks:* Uses `::operator delete` ([[new.delete]]), but it is
 unspecified when this function is called.
-#### `allocator` globals <a id="allocator.globals">[[allocator.globals]]</a>
 ``` cpp
 template<class T, class U>
-  bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
 ```
 *Returns:* `true`.
-``` cpp
-template <class T, class U>
-  bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;
-```
-*Returns:* `false`.

 ### The default allocator <a id="default.allocator">[[default.allocator]]</a>
+All specializations of the default allocator meet the allocator
+completeness requirements [[allocator.requirements.completeness]].
 ``` cpp
 namespace std {
   template<class T> class allocator {
    public:
     using value_type                             = T;
+    using size_type                              = size_t;
+    using difference_type                        = ptrdiff_t;
     using propagate_on_container_move_assignment = true_type;
     using is_always_equal                        = true_type;
+ constexpr allocator() noexcept;
+ constexpr allocator(const allocator&) noexcept;
+    template<class U> constexpr allocator(const allocator<U>&) noexcept;
+ constexpr ~allocator();
+    constexpr allocator& operator=(const allocator&) = default;
+ [[nodiscard]] constexpr T* allocate(size_t n);
+ constexpr void deallocate(T* p, size_t n);
   };
 }
 ```
+#### Members <a id="allocator.members">[[allocator.members]]</a>
 Except for the destructor, member functions of the default allocator
+shall not introduce data races [[intro.multithread]] as a result of
 concurrent calls to those member functions from different threads. Calls
 to these functions that allocate or deallocate a particular unit of
 storage shall occur in a single total order, and each such deallocation
 call shall happen before the next allocation (if any) in this order.
 ``` cpp
+[[nodiscard]] constexpr T* allocate(size_t n);
 ```
+*Mandates:* `T` is not an incomplete type [[basic.types]].
+*Returns:* A pointer to the initial element of an array of `n` `T`.
+*Remarks:* The storage for the array is obtained by calling
+`::operator new` [[new.delete]], but it is unspecified when or how often
+this function is called. This function starts the lifetime of the array
+object, but not that of any of the array elements.
+*Throws:* `bad_array_new_length` if
+`numeric_limits<size_t>::max() / sizeof(T) < n`, or `bad_alloc` if the
+storage cannot be obtained.
 ``` cpp
+constexpr void deallocate(T* p, size_t n);
 ```
+*Preconditions:* `p` is a pointer value obtained from `allocate()`. `n`
+equals the value passed as the first argument to the invocation of
 allocate which returned `p`.
 *Effects:* Deallocates the storage referenced by `p` .
+*Remarks:* Uses `::operator delete` [[new.delete]], but it is
 unspecified when this function is called.
+#### Operators <a id="allocator.globals">[[allocator.globals]]</a>
 ``` cpp
 template<class T, class U>
+ constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
 ```
 *Returns:* `true`.

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