Extended Description

Consider this code:

class span {
public:
using pointer = const int *;

constexpr span() : __data{nullptr} {}

constexpr span           (const span&) noexcept = default;
constexpr span& operator=(const span&) noexcept = default;
~span() noexcept = default;

constexpr pointer data()    const noexcept { return __data; }

private:
pointer __data;
};

When compiled with a recent clang [clang version 7.0.0 (trunk 336548)], (-std=c++2a) it gives an error:
junk.cpp:8:5: error: defaulted definition of copy assignment operator is not
constexpr
constexpr span& operator=(const span&) noexcept = default;
^

However, if I switch the order of the two lines to:
~span() noexcept = default;
constexpr span& operator=(const span&) noexcept = default;

then it compiles w/o error.

However, I believe that it should compile w/o error in both cases.