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Section: 20.10.10 [default.allocator] Status: WP Submitter: Jonathan Wakely Opened: 2019-10-25 Last modified: 2020-02-24
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In C++20 the std::allocator<void> explicit specialization is gone, which means it uses the primary template, which has allocate and deallocate members.Although it's implied by the use of sizeof(T), std::allocator<T>::allocate doesn't have an explicit precondition that the value type is complete.
[2019-11 Status to 'Ready' in Monday issue prioritization in Belfast]
This wording is relative to N4835.
Modify 188.8.131.52 [allocator.members] as indicated:
[[nodiscard]] constexpr T* allocate(size_t n);
-2- Returns: A pointer to the initial element of an array of storage of size n * sizeof(T), aligned appropriately for objects of type T. […]