P2952R1
auto& operator=(X&&) = default

1. Changelog

• R1:

  • Add discussion of void operator=(X&&) = default, and of triviality, in § 3.5 Other return types are still forbidden.

2. Motivation

Current C++ permits =default to appear only on certain signatures, with certain return types. The current wording prohibits the use of placeholder types such as auto& to express these return types, with the single exception of C++20’s operator<=>. This leads to redundant repetition, such as in this real code from libc++'s test suite:

struct ForwardDiffView {
  [...]
  ForwardDiffView(ForwardDiffView&&) = default;
  ForwardDiffView(const ForwardDiffView&) = default;
  ForwardDiffView& operator=(ForwardDiffView&&) = default;
  ForwardDiffView& operator=(const ForwardDiffView&) = default;
  [...]
};

struct ForwardDiffView {
  [...]
  ForwardDiffView(ForwardDiffView&&) = default;
  ForwardDiffView(const ForwardDiffView&) = default;
  auto& operator=(ForwardDiffView&&) = default;
  auto& operator=(const ForwardDiffView&) = default;
  [...]
};

The comparison operators are inconsistent among themselves: operator<=> can deduce strong_ordering, but the others cannot deduce bool.

auto operator<=>(const MyClass& rhs) const = default;
bool operator==(const MyClass& rhs) const = default;
bool operator<(const MyClass& rhs) const = default;

auto operator<=>(const MyClass& rhs) const = default;
auto operator==(const MyClass& rhs) const = default;
auto operator<(const MyClass& rhs) const = default;

The status quo is inconsistent between non-defaulted and defaulted functions, making it unnecessarily tedious to upgrade to =default:

auto& operator=(const MyClass& r) { i = r.i; return *this; }
MyClass& operator=(const MyClass& r) = default;

auto& operator=(const MyClass& r) { i = r.i; return *this; }
auto& operator=(const MyClass& r) = default;

auto operator==(const MyClass& r) const { return i == r.i; }
bool operator==(const MyClass& r) const = default;

auto operator==(const MyClass& r) const { return i == r.i; }
auto operator==(const MyClass& r) const = default;

The ill-formedness of these declarations comes from overly restrictive wording in the standard, such as [class.eq]/1 specifically requiring that a defaulted equality operator must have a declared return type of bool, instead of simply specifying that its return type must be bool. We believe each of the examples above has an intuitively clear meaning: the placeholder return type correctly matches the type which the defaulted body will actually return. We propose to loosen the current restrictions and permit these declarations to be well-formed.

This proposal does not seek to change the set of valid return types for these functions. We propose a purely syntactic change to expand the range of allowed declaration syntax, not semantics. (But we do one drive-by clarification which we believe matches EWG’s original intent: if an empty class’s defaulted operator<=> returns a non-comparison-category type, it should be defaulted as deleted.)

3. Proposal

We propose that a defaulted function declaration with a placeholder return type should have its type deduced ([dcl.spec.auto.general]) as if from a fictional return statement that returns:

• a prvalue of type bool, in the case of operator==, operator!=, operator<, operator>, operator<=, or operator>=

• a prvalue of type Q, in the case of operator<=>; where Q is the common comparison type of R₀, R₁,... R_n-1

• an lvalue of type C, in the case of operator= for a class or union type C

Then, the deduced return type is compared to the return type(s) permitted by the standard. If the types match, the declaration is well-formed. Otherwise it’s ill-formed.

struct MyClass {
  auto& operator=(const MyClass&) = default;          // Proposed OK: deduces MyClass&
  decltype(auto) operator=(const MyClass&) = default; // Proposed OK: deduces MyClass&
  auto&& operator=(const MyClass&) = default;         // Proposed OK: deduces MyClass&
  const auto& operator=(const MyClass&) = default;    // Still ill-formed: deduced const MyClass& is not MyClass&
  auto operator=(const MyClass&) = default;           // Still ill-formed: deduced MyClass is not MyClass&
  auto* operator=(const MyClass&) = default;          // Still ill-formed: deduction fails
  void operator=(const MyClass&) = default;           // Still ill-formed: void is not MyClass&
};

struct MyClass {
  auto operator==(const MyClass&) const = default;           // Proposed OK: deduces bool
  decltype(auto) operator==(const MyClass&) const = default; // Proposed OK: deduces bool
  auto&& operator==(const MyClass&) const = default;         // Still ill-formed: deduced bool&& is not bool
  auto& operator==(const MyClass&) const = default;          // Still ill-formed: deduction fails
};

3.1. "Return type" versus "declared return type"

Today, vendors unanimously reject auto& operator=(const A&) = default. But we can’t find any wording in [class.copy.assign] or [dcl.fct.def.default] that directly justifies this behavior. It seems that vendors are interpreting e.g. [dcl.fct.def.default]/2.5’s "[if] the return type of F₁ differs from the return type of F₂" to mean "the declared return type of F₁," even though newer sections such as [class.compare] consistently distinguish the "declared return type" from the (actual) return type.

We tentatively propose to leave [dcl.fct.def.default] alone, and simply add an example that indicates the (new) intent of the (existing) wording: that it should now be interpreted as talking about the assignment operator’s actual return type, not its declared (placeholder) return type.

3.2. "Defaulted as deleted"

The current wording for comparison operators is crafted so that the following Container is well-formed. Its operator<=> is defaulted as deleted (so that operator is unusable), but the instantiation of class Container<mutex> itself is OK. We need to preserve this in our rewriting. (Godbolt.)

template<class T>
struct Container {
  T t;
  auto operator<=>(const Container&) const = default;
};

Container<std::mutex> cm;
  // OK, <=> is deleted

struct Weird { int operator<=>(Weird) const; };
Container<Weird> cw;
  // OK, <=> is deleted because Weird’s operator<=>
  // returns a non-comparison-category type

Similarly for dependent return types:

template<class R>
struct C {
  int i;
  R operator<=>(const C&) const = default;
};
static_assert(std::three_way_comparable<C<std::strong_ordering>>);
static_assert(!std::three_way_comparable<C<int>>);
  // OK, C<int>'s operator<=> is deleted

Therefore we can’t just say "operator<=> shall have a return type which is a comparison category type"; we must say that if the return type is not a comparison category type then the operator is defaulted as deleted.

3.3. "Deducing this" and CWG2586

The resolution of [CWG2586] (adopted for C++23) permits defaulted functions to have explicit object parameters. This constrains the wordings we can choose for operator=: we can’t say “the return type is deduced as if from return *this” because there might not be a *this.

There’s a quirk with rvalue-ref-qualified assignment operators — not move assignment, but assignment where the destination object is explicitly rvalue-ref-qualified.

• auto&& operator=(const B&) && { return *this; } deduces an lvalue reference; but

• auto&& operator=(this B&& self, const B&) { return self; } deduces an rvalue reference.

Nonetheless, a defaulted assignment operator always returns an lvalue reference ([class.copy.assign]/6, [dcl.fct.def.default]/2.5), regardless of whether it’s declared using explicit object syntax.

struct A {
  A& operator=(const A&) && = default; // OK today
  A&& operator=(const A&) && = default; // Ill-formed, return type isn’t A&
  decltype(auto) operator=(const A&) && { return *this; } // OK, deduces A&
  decltype(auto) operator=(const A&) && = default; // Proposed OK, deduces A&
};
struct B {
  B& operator=(this B&& self, const B&) { return self; } // Error, self can’t bind to B&
  B&& operator=(this B&& self, const B&) { return self; } // OK
  decltype(auto) operator=(this B&& self, const B&) { return self; } // OK, deduces B&&
  B& operator=(this B& self, const B&) = default; // OK
  B& operator=(this B&& self, const B&) = default; // OK
  B&& operator=(this B&& self, const B&) = default; // Ill-formed, return type isn’t B&
  decltype(auto) operator=(this B&& self, const B&) = default; // Proposed OK, deduces B&
};

Defaulted rvalue-ref-qualified assignment operators are weird; Arthur is bringing another paper to forbid them entirely ([P2953]). However, P2952 doesn’t need to treat them specially. Defaulted assignment operators invariably return lvalue references, so we invariably deduce as-if-from an lvalue reference, full stop.

3.4. Burden on specifying new defaultable operators

We propose to leave [dcl.fct.def.default] alone and reinterpret its term "return type" to mean the actual return type, not the declared return type. This will, by default, permit the programmer to use placeholder return types on their defaulted operators. So there is a new burden on the specification of the defaultable operator, to specify exactly how return type deduction works for the implicitly defined operator.

struct add_postfix_increment {
    template<class Self>
    auto operator++(this Self&, int) = default;
      // Today: ill-formed, can’t default operator++
      // After P1046R2: presumably still not OK, can’t default a template
};
struct S : add_postfix_increment {
    int i;
    auto& operator++() { ++i; return *this; }
    using add_postfix_increment::operator++;
};
S s = {1};
S t = s++;

3.5. Other return types are still forbidden

Notice that returning any type other than X& from a defaulted X::operator= remains ill-formed. For example, we don’t propose to accept:

struct X {
  void operator=(X&&) = default; // still ill-formed
};

Any paper that did propose to permit defaulting void operator= would have to consider very carefully whether to call that operator "trivial." Today, is_trivially_move_assignable_v<T> is true only when decltype(declval<T&>() = declval<T>()) is T&. It’s just barely conceivable that existing user code might rely on that fact, and break when presented with a trivial assignment operator that returned void instead. And certainly the return-by-copy X operator=(X&&) = default shouldn’t be considered "trivial" for non-trivially-copy-constructible X.

But none of this is a problem for this proposal P2952, because we propose to continue rejecting a defaulted X::operator= that returns any type but X&. We merely propose to allow the programmer to spell that return type using a placeholder such as auto&.

3.6. Existing corner cases

There is vendor divergence in some corner cases. Here is a table of the divergences we found, plus our opinion as to the conforming behavior, and our proposed behavior.

const bool operator==(const C&) const = default;

friend bool operator==(const C, const C) = default;

decltype(auto) operator<=>(const C&) const = default;

const auto operator<=>(const C&) const = default;

True auto operator<=>(const C&) const = default;

False auto operator<=>(const C&) const = default;

struct U { U(std::strong_ordering); };
struct C {
  U operator<=>(const C&) const = default;
};

struct U { U(std::strong_ordering); operator int(); };
struct C {
  int i;
  U operator<=>(const C&) const = default;
};

struct C {
  int i;
  const std::strong_ordering&
    operator<=>(const C&) const = default;
};

struct C {
  const std::strong_ordering&
    operator<=>(const C&) const = default;
};

struct W {
  const std::strong_ordering
    operator<=>(const W&) const;
};
struct C {
  W w;
  auto operator<=>(const C&) const = default;
};

struct W {
  const std::strong_ordering
    operator<=>(const W&) const;
};
struct C {
  W w;
  std::strong_ordering
    operator<=>(const C&) const = default;
};

auto operator<=>(const M&) const
  noexcept(false) = default;

C& operator=(C&) = default;

C& operator=(const C&&) = default;

C& operator=(const C&) const = default;

C& operator=(const C&) && = default;

C&& operator=(const C&) && = default;

3.7. Impact on existing code

There should be little effect on existing code, since this proposal mainly allows syntax that was ill-formed before. As shown in § 3.6 Existing corner cases, we do propose to change some very arcane examples, e.g.

struct C {
  const std::strong_ordering&
    operator<=>(const C&) const = default;
    // Today: Well-formed, non-deleted
    // Tomorrow: Well-formed, deleted
};

4. Implementation experience

None yet.

5. Proposed wording

5.1. [class.eq]

Note: The phrase "equality operator function" ([over.binary]) covers both == or !=. But != is not covered by [class.eq]; it’s covered by [class.compare.secondary] below.

Modify [class.eq] as follows:

1․ A defaulted equality operator function ([over.binary]) shall have a declared return type bool.

2․ A defaulted == operator function for a class C is defined as deleted unless, for each x_i in the expanded list of subobjects for an object x of type C, x_i == x_i is usable ([class.compare.default]).

3․ The return value V of a defaulted == operator function with parameters x and y is determined by comparing corresponding elements x_i and y_i in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where x_i == y_i yields a result value which , when contextually converted to bool, yields false. If no such index exists, V is true. Otherwise, V is false.

x․ A defaulted == operator function shall have the return type bool. If its declared return type contains a placeholder type, its return type is deduced as if from return true;.

4․ [Example 1:

struct D {
  int i;
  friend bool operator==(const D& x, const D& y) = default;
      // OK, returns x.i == y.i
};

— end example]

5.2. [class.spaceship]

Note: There are only three "comparison category types" in C++, and strong_ordering::equal is implicitly convertible to all three of them. The status quo already effectively forbids <=> to return a non-comparison-category type, since either R is deduced as a common comparison type (which is a comparison category type by definition), or else a synthesized three-way comparison of type R must exist (which means R must be a comparison category type), or else the sequence x_i must be empty (in which case there are no restrictions on R except that it be constructible from strong_ordering::equal). We strengthen the wording to directly mandate that the return type be a comparison category type, even in the empty case.

Modify [class.spaceship] as follows:

[...]

2․ Let R be the declared return type of a defaulted three-way comparison operator function, and let x_i be the elements of the expanded list of subobjects for an object x of type C.

— (2.1) If R is auto, contains a placeholder type, then let cv_i R_i be the type of the expression x_i <=> x_i. The operator function is defined as deleted if that expression is not usable or if R_i is not a comparison category type ([cmp.categories.pre]) for any i. The return type is deduced as if from return Q(std::strong_ordering::equal);, where Q is the common comparison type (see below) of R₀, R₁, ..., R_n-1.

— (2.2) Otherwise, R shall not contain a placeholder type. If if the synthesized three-way comparison of type R between any objects x_i and x_i is not defined, the operator function is defined as deleted.

3․ The return value V of type R of the defaulted three-way comparison operator function with parameters x and y of the same type is determined by comparing corresponding elements x_i and y_i in the expanded lists of subobjects for x and y (in increasing index order) until the first index i where the synthesized three-way comparison of type R between x_i and y_i yields a result value v_i where v_i != 0, contextually converted to bool, yields true; V is a copy of v_i. If no such index exists, V is static_cast<R>(std::strong_ordering::equal).

x․ A defaulted three-way comparison operator function which is not deleted shall have a return type which is a comparison category type ([cmp.categories.pre]).

4․ The common comparison type U of a possibly-empty list of n comparison category types T₀, T₁, ..., T_n-1 is defined as follows:

[...]

5.3. [class.compare.secondary]

Modify [class.compare.secondary] as follows:

1․ A secondary comparison operator is a relational operator ([expr.rel]) or the != operator.

A defaulted operator function ([over.binary]) for a secondary comparison operator @ shall have a declared return type bool.

2․ The A defaulted secondary comparison operator function with parameters x and y is defined as deleted if

— (2.1) overload resolution ([over.match]), as applied to x @ y, does not result in a usable candidate, or

— (2.2) the candidate selected by overload resolution is not a rewritten candidate.

Otherwise, the operator function yields x @ y. The defaulted operator function is not considered as a candidate in the overload resolution for the @ operator.

x․ A defaulted secondary comparison operator function shall have the return type bool. If its declared return type contains a placeholder type, its return type is deduced as if from return true;.

3․ [Example 1:

struct HasNoLessThan { };

struct C {
  friend HasNoLessThan operator<=>(const C&, const C&);
  bool operator<(const C&) const = default; // OK, function is deleted
};

— end example]

5.4. [class.copy.assign]

Note: [class.copy.assign]/6 already clearly states that "The implicitly-declared copy/move assignment operator for class X has the return type X&." But we need this new wording to ensure that an explicitly-defaulted copy/move assignment operator will deduce that same type. (If it didn’t deduce that type, then the explicitly-defaulted operator would be deleted, as in example B below.)

Note: Arthur initially proposed that [class.copy.assign]/14 should say "...returns an lvalue reference to the object for which...", but Jens Maurer thought that wouldn’t be an improvement from CWG’s point of view. Maybe it should say "...returns the assignment operator’s object parameter ([dcl.fct])" instead.

Modify [class.copy.assign] as follows:

14․ The implicitly-defined copy/move assignment operator for a class returns the object for which the assignment operator is invoked, that is, the object assigned to.

15․ If a defaulted copy/move assignment operator’s declared return type contains a placeholder type, its return type is deduced as if from return r;, where r is an lvalue reference to the object for which the assignment operator is invoked.

16․ [Example:

struct A {
  decltype(auto) operator=(A&&) = default;
    // Return type is A&
};
struct B {
  auto operator=(B&&) = default;
    // error: Return type is B, which violates [dcl.fct.def.default]/2.5
};

—end example]