trunk regression: `operator==` is not defined

[Fedr]

This C++20 program

#include <compare>

struct A {
    friend constexpr auto operator<=>(const A &, const A &) = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;

static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );

is accepted in GCC and Clang 18.1, but the current Clang trunk rejects it with the error

<source>:9:16: error: static assertion expression is not an integral constant expression
    9 | static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:16: note: undefined function 'operator==' cannot be used in a constant expression
<source>:4:27: note: declared here
    4 |     friend constexpr auto operator<=>(const A &, const A &) = default;
      |                           ^

Online demo: https://gcc.godbolt.org/z/ssqxsoYac

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Fedor Chelnokov (Fedr)

This C++20 program ``` #include <compare>

struct A {
friend constexpr auto operator<=>(const A &, const A &) = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;

static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );

is accepted in GCC and Clang 18.1, but the current Clang trunk rejects it with the error

<source>:9:16: error: static assertion expression is not an integral constant expression
9 | static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:16: note: undefined function 'operator==' cannot be used in a constant expression
<source>:4:27: note: declared here
4 | friend constexpr auto operator<=>(const A &, const A &) = default;
| ^

Online demo: https://gcc.godbolt.org/z/ssqxsoYac
</details>

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Fedor Chelnokov (Fedr)

This C++20 program ``` #include <compare>

struct A {
friend constexpr auto operator<=>(const A &, const A &) = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;

static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );

is accepted in GCC and Clang 18.1, but the current Clang trunk rejects it with the error

<source>:9:16: error: static assertion expression is not an integral constant expression
9 | static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:9:16: note: undefined function 'operator==' cannot be used in a constant expression
<source>:4:27: note: declared here
4 | friend constexpr auto operator<=>(const A &, const A &) = default;
| ^

Online demo: https://gcc.godbolt.org/z/ssqxsoYac
</details>

[MitalAshok]

This happens with the Clang 19 release candidate too so appears to be a Clang 19/20 regression.

Bisect points to d4cf20c / #91339, which seems reasonable.

CC @sdkrystian

[sdkrystian]

FWIW, clang 18.1 will reject if operator== is explicitly defined as defaulted within the class definition:

struct A {
    friend constexpr auto operator<=>(const A &, const A &) = default;
    friend constexpr bool operator==(const A &, const A &) = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;

static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );

[shafik]

FWIW, clang 18.1 will reject if operator== is explicitly defined as defaulted within the class definition:

struct A {
    friend constexpr auto operator<=>(const A &, const A &) = default;
    friend constexpr bool operator==(const A &, const A &) = default;
};

constexpr bool operator ==(const A &, const A &) noexcept;

static_assert( ((bool(*)(const A &, const A &))&operator==)( A{}, A{} ) );

Are you saying this is not a bug or not a regression?