Why is unique_ptr not equality_comparable_with nullptr_t in C++20?

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Working with C++20's concepts I noticed that std::unique_ptr appears to fail to satisfy the std::equality_comparable_with<std::nullptr_t,...> concept. From std::unique_ptr's definition, it is supposed to implement the following when in C++20:

template<class T1, class D1, class T2, class D2>
bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

template <class T, class D>
bool operator==(const unique_ptr<T, D>& x, std::nullptr_t) noexcept;

This requirement should implement symmetric comparison with nullptr -- which from my understanding is sufficient for satisfying equality_comparable_with.

Curiously, this issue appears to be consistent on all the major compilers. The following code is rejected from Clang, GCC, and MSVC:

// fails on all three compilers
static_assert(std::equality_comparable_with<std::unique_ptr<int>,std::nullptr_t>);

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However the same assertion with std::shared_ptr is accepted:

// succeeds on all three compilers
static_assert(std::equality_comparable_with<std::shared_ptr<int>,std::nullptr_t>);

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Unless I'm misunderstanding something, this appears to be a bug. My question is whether this is a coincidental bug in the three compiler implementations, or is this a defect in the C++20 standard?

Note: I'm tagging this language-lawyer in case this happens to be a defect.

c++ language-lawyer c++20 c++-concepts
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    "which from my understanding is sufficient for satisfying equality_comparable_with." It is not, but I don't see any other requirements that aren't satisfied. – Nicol Bolas Apr 4 at 4:29

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