default equality operator== const selects not-const conversion operator

[Fedr]

This program

struct A {
    int i = 0;
    constexpr operator int() const { return i; }
    constexpr operator int&() { return ++i; }
};

struct B : A {
    bool operator==(const B &) const = default;
};

constexpr bool f() {
    B x;
    return x == x;
}

static_assert( f() ); // fails in Clang

is accepted by GCC and MSVC, but fails static assertion in Clang. Online demo: https://gcc.godbolt.org/z/5hYob3K66

Clang looks wrong here, because not-const operator int& should not be called from within B::operator==(const B &) const.

Related discussion: https://stackoverflow.com/q/78850335/7325599

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Fedor Chelnokov (Fedr)

This program ``` struct A { int i = 0; constexpr operator int() const { return i; } constexpr operator int&() { return ++i; } };

struct B : A {
bool operator==(const B &) const = default;
};

constexpr bool f() {
B x;
return x == x;
}

static_assert( f() ); // fails in Clang

is accepted by GCC and MSVC, but fails static assertion in Clang. Online demo: https://gcc.godbolt.org/z/5hYob3K66

Clang looks wrong here, because not-const `operator int&` should not be called from within `B::operator==(const B &) const`.

Related discussion: https://stackoverflow.com/q/78850335/7325599
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