operator<=> doesn’t obsolete the hidden friend idiom

C++20 introduces new expression-rewriting rules for comparison operators, such that the compiler can rewrite a < b as either (a <=> b) < 0 or 0 < (b <=> a), and a != b as either !(a == b) or !(b == a). Today, someone asked whether these new rules decrease the attractiveness of the hidden friend idiom for overloaded operators — at least for operator<=>, should we just declare it as a member function? The answer is no. Observe (Godbolt):

struct Good {
    friend auto operator<=>(const Good&, const Good&) = default;

struct Bad {
    auto operator<=>(const Bad&) const = default;


static_assert(not std::totally_ordered<std::reference_wrapper<Bad>>); // !!

Consider an expression such as

Good g;
auto rg = std::ref(g);
auto og = rg < rg;  // OK

The compiler will rewrite rg < rg into (rg <=> rg) < 0, and then use ADL to look up a declaration for <=> in the places associated with std::reference_wrapper<Good>. (See “What is ADL?” (2019-04-26).) ADL will find the hidden friend of Good. Both operands have a user-defined implicit conversion to const Good&, so the hidden friend is viable; and so everything’s fine.

But in the Bad case…

Bad b;
auto rb = std::ref(b);
auto ob = rb < rb;  // ill-formed

The compiler does the same rewrite into (rb <=> rb) < 0, but this time, ADL doesn’t find any friends of Bad. Lookup also finds no member functions named operator<=>, because std::reference_wrapper<Bad> has no such member functions. (The member functions of Bad itself are not consulted, because neither operand is actually of type Bad.)

The hidden friend idiom has some engineering benefits as well, such as reducing the size of the overload set the compiler sees (that’s the “hidden” part) and making it harder to forget to apply const symmetrically to both operands. So it’s just all around a good idea. The point of this post is simply to show that it also has a real effect on your types’ observable API. C++20’s new expression-rewriting rules don’t change that fact.

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Posted 2021-10-22