[BUG]: cuda::std::swap cannot ADL-two-step-swap a type from std

[bernhardmgruber]

Is this a duplicate?

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Type of Bug

Compile-time Error

Component

libcu++

Describe the bug

While trying to write a function in Thrust that needs to swap allocators from std and thrust, I ran into the issue that cuda::std::swap fails to swap them.

How to Reproduce

#include <thrust/device_vector.h>
#include <utility>

void swap_allocators(auto& a, auto& b) {
    using cuda::std::swap;
    swap(a, b);
}

int main() {
    thrust::device_vector<int>::allocator_type a, b;
    swap_allocators(a, b); // works

    std::allocator<int> c, d;
    swap_allocators(c, d); // /fails
}

https://godbolt.org/z/Ea1d1TWdo

Errors with:

<source>(6): error: more than one instance of overloaded function "swap" matches the argument list:
... 

I think this should work, since the ADL-two-step call is required to pick up any user-provided swap overload.

Expected behavior

Compile successfully.

Reproduction link

No response

Operating System

No response

nvidia-smi output

No response

NVCC version

No response

[miscco]

There is nothing we can do here.

The issue is that ADL will always pull in std::swap. If you tell it to also pull in cuda::std::swap that will always be ambiguous.

Nothing we can do here the moment something pulls in namespace std

[bernhardmgruber]

There is nothing we can do here.

So, how do we swap two objects in a generic context? Like swapping two allocators which could come from std::, thrust::, cuda::std::, or cuda::? How do I utter the call to swap (and to which one)?