Issue 3175: The CommonReference requirement of concept SwappableWith is not satisfied in the example (original) (raw)

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3175. The CommonReference requirement of concept SwappableWith is not satisfied in the example

Section: 18.4.9 [concept.swappable] Status: C++20 Submitter: Kostas Kyrimis Opened: 2018-12-14 Last modified: 2021-02-25

Priority: 1

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Discussion:

The defect stems from the example found in sub-clause 18.4.9 [concept.swappable] p5:

[…]

template<class T, std::SwappableWith U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward(t), std::forward(u)); }

[…] namespace N { struct A { int m; }; struct Proxy { A* a; }; Proxy proxy(A& a) { return Proxy{ &a }; }

void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement }

int main() { […] N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); // diagnostic manifests here(#1) assert(a1.m == -5 && a2.m == 5); }

The call to value_swap(a1, proxy(a2)) resolves to [T = N::A&, U = N::Proxy] The compiler will issue a diagnostic for #1 because:

  1. rvalue proxy(a2) is not swappable
  2. concept SwappableWith<T, U> requires N::A and Proxy to modelCommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> It follows from the example that there is no common reference for [T = N::A&, U = N::Proxy]

[2019-06-20; Casey Carter comments and provides improved wording]

The purpose of the CommonReference requirements in the cross-type concepts is to provide a sanity check. The fact that two types satisfy a single-type concept, have a common reference type that satisfies that concept, and implement cross-type operations required by the cross-type flavor of that concept very strongly suggests the programmer intends them to model the cross-type concept. It's an opt-in that requires some actual work, so it's unlikely to be inadvertent.

The CommonReference<const T&, const U&> pattern makes sense for the comparison concepts which require that all variations of const and value category be comparable: we use const lvalues to trigger the "implicit expression variation" wording in 18.2 [concepts.equality]. SwappableWith, however, doesn't care about implicit expression variations: it only needs to witness that we can exchange the values denoted by two reference-y expressions E1 and E2. This suggests that CommonReference<decltype((E1)), decltype((E2))> is a more appropriate requirement than the current CommonReference<const remove_reference_t<…> mess that was blindly copied from the comparison concepts.

We must change the definition of "exchange the values" in 18.4.9 [concept.swappable] — which refers to the common reference type — consistently.

Previous resolution [SUPERSEDED]:

This wording is relative to N4791.

  1. Change 18.4.9 [concept.swappable] as indicated:

    -3- […]

    template
    concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); };

    template<class T, class U>
    concept SwappableWith =
    CommonReference<T, Uconst remove_reference_t&, const remove_reference_t&> &&
    requires(T&& t, U&& u) {
    ranges::swap(std::forward(t), std::forward(t));
    ranges::swap(std::forward(u), std::forward(u));
    ranges::swap(std::forward(t), std::forward(u));
    ranges::swap(std::forward(u), std::forward(t));
    };

    -4- […]

    -5- [Example: User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:

    #include

#include
#include

namespace ranges = std::ranges;

template<class T, std::SwappableWith U>
void value_swap(T&& t, U&& u) {
ranges::swap(std::forward(t), std::forward(u));
}

template<std::Swappable T>
void lv_swap(T& t1, T& t2) {
ranges::swap(t1, t2);
}

namespace N {
struct A { int m; };
struct Proxy {
A* a;
Proxy(A& a) : a{&a} {}
friend void swap(Proxy&& x, Proxy&& y) {
ranges::swap(x.a, y.a);
}
};
Proxy proxy(A& a) { return Proxy{ &a }; }
void swap(A& x, Proxy p) {
ranges::swap(x.m, p.a->m);
}
void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
}

int main() {
int i = 1, j = 2;
lv_swap(i, j);
assert(i == 2 && j == 1);
N::A a1 = { 5 }, a2 = { -5 };
value_swap(a1, proxy(a2));
assert(a1.m == -5 && a2.m == 5);
}

[2020-01-16 Priority set to 1 after discussion on the reflector.]

[2020-02-10 Move to Immediate Monday afternoon in Prague]

Proposed resolution:

This wording is relative to N4820.

  1. Change 18.4.9 [concept.swappable] as indicated:

    -1- Let t1 and t2 be equality-preserving expressions that denote distinct equal objects of type T, and let u1 and u2 similarly denote distinct equal objects of type U. [Note: t1 and u1 can denote distinct objects, or the same object. — _end note_] An operation exchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:

    1. (1.1) — If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
    2. (1.2) — If T and U are different types that model CommonReference<const T&, const U&>and CommonReference<decltype((t1)), decltype((u1))> is modeled, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_reference_t<~~const T&, const U&~~decltype((t1)), decltype((u1))>.

    -2- […]

    -3- […]

    template
    concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); };

    template<class T, class U>
    concept SwappableWith =
    CommonReference<T, Uconst remove_reference_t&, const remove_reference_t&> &&
    requires(T&& t, U&& u) {
    ranges::swap(std::forward(t), std::forward(t));
    ranges::swap(std::forward(u), std::forward(u));
    ranges::swap(std::forward(t), std::forward(u));
    ranges::swap(std::forward(u), std::forward(t));
    };

    -4- […]

    -5- [Example: User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows:

    #include

#include
#include

namespace ranges = std::ranges;

template<class T, std::SwappableWith U>
void value_swap(T&& t, U&& u) {
ranges::swap(std::forward(t), std::forward(u));
}

template<std::Swappable T>
void lv_swap(T& t1, T& t2) {
ranges::swap(t1, t2);
}

namespace N {
struct A { int m; };
struct Proxy {
A* a;
Proxy(A& a) : a{&a} {}
friend void swap(Proxy x, Proxy y) {
ranges::swap(*x.a, *y.a);
}
};
Proxy proxy(A& a) { return Proxy{ &a }; }
void swap(A& x, Proxy p) {
ranges::swap(x.m, p.a->m);
}
void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement
}

int main() {
int i = 1, j = 2;
lv_swap(i, j);
assert(i == 2 && j == 1);
N::A a1 = { 5 }, a2 = { -5 };
value_swap(a1, proxy(a2));
assert(a1.m == -5 && a2.m == 5);
}