Explicit calls to `std::swap` are made on user provided template types. This does not work for `C++20`.

[melton1968]

Starting with C++20, fully specializing a function template in the std namespace is undefined behavior. This precludes implementing swap for user-defined types in the standard namespace. You can find the reasoning in this paper.

I am running into this issue while using boost::sort in a project using C++20. I have implemented a free function swap for my type in the type's namespace, but since the library code is explicitly calling std::swap, it cannot find it. If I implement the free function in std, the compiler chooses to ignore it, which is allowed under the standard.

I changed the sort code to use the "use std::swap and then call swap unqualified" idiom, and everything worked as expected. For example:

std::swap(*iter1, *iter2);

becomes,

using std::swap;
swap(*iter1, *ite2);

I can submit a PR for this change if that is appropriate.

[spreadsort]

On Tue, Jun 6, 2023, 5:04 PM Mark Melton @.***> wrote:

Starting with C++20, fully specializing a function template https://en.cppreference.com/w/cpp/language/extending_std in the std namespace is undefined behavior. This precludes implementing swap for user-defined types in the standard namespace. You can find the reasoning in this paper https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0551r0.pdf.

I am running into this issue while using boost::sort in a project using C++20. I have implemented a free function swap for my type in the type's namespace, but since the library code is explicitly calling std::swap, it cannot find it. If I implement the free function in std, the compiler chooses to ignore it, which is allowed under the standard.

I changed the sort code to use the "use std::swap and then call swap unqualified" idiom, and everything worked as expected. For example:

std::swap(iter1, iter2);

becomes,

using std::swap;swap(iter1, ite2);

I can submit a PR for this change if that is appropriate.

Yes, please.

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[nigels-com]

Resolved in PR #73, by the look of it.

[spreadsort]

Thanks Nigel!