duplicate of #26063 ?

Still persists in Clang 13 with constexpr constructors.

This issue doesn't appear in GCC 11.2 and Clang 13 with -std=c++17, but it only appears when the flag -std=c++20 is given.

Below is the sample code:

struct Foo {
  constexpr Foo(const Foo&) noexcept = default;
  constexpr Foo(Foo&&) noexcept = default;

  constexpr auto operator=(const Foo&) noexcept -> Foo& = default;
  constexpr auto operator=(Foo&&) noexcept -> Foo& = default;
};

with the following errors:

<source>:5:3: error: defaulted definition of copy assignment operator is not constexpr
  constexpr auto operator=(const Foo&) noexcept -> Foo& = default;
  ^
<source>:6:3: error: defaulted definition of move assignment operator is not constexpr
  constexpr auto operator=(Foo&&) noexcept -> Foo& = default;
  ^
2 errors generated.
Compiler returned: 1

Live demo: https://godbolt.org/z/6Mz6K3PMf

As shown above, the warning states that both assignment operators are not constexpr even though they are already declared constexpr, which is contradictory.

The original issue was a duplicate.

Still persists in Clang 13 with constexpr constructors.

No. Lack of the constexpr default constructor was critical before C++23, see N4868 [basic.type]/10.5.2 and P2448R2.

Clang was correct on rejecting constexpr on Foo::operator=, but the diagnostic message was wrong. #85145 fixed the diagnostic message in Clang 19.

Now Clang says

error: defaulted definition of copy assignment operator cannot be marked constexpr before C++23

Godbolt link

@llvm/issue-subscribers-clang-frontend

Author: Marshall Clow (mclow)

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;
};

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;