[Bug c++/67796] New: Definition for custom std::swap found by ADL but not used

[rkirchge at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67796

Bug ID: 67796
   Summary: Definition for custom std::swap found by ADL but not
used
   Product: gcc
   Version: 5.2.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: rkirchge at gmail dot com
  Target Milestone: ---

I am trying to use ADL to find a custom implementation of swap for tuples of an
reference types. I am compiling with "-std=c++14 -Wall -Wextra".

Without the following definition, the compiler reports that it cannot find the
function required. With the code, the compilation succeeds without warning but
"ADL works" does not print.

namespace std { 
template 
void swap(std::tuple lhs, std::tuple rhs) noexcept
{  
  std::cout<<"ADL works!"< tmp = lhs;   
  lhs = rhs;
  rhs = tmp;   
   }

>From my tests the swap function being used when I include the above code
behaves as:
auto tmp = lhs;
lhs = rhs;
rhs = lhs; 

But since these are std::tuples of reference types, the temporary value created
is also  a tuple of reference types, so it does not work as I intend.

[Bug c++/67796] Definition for custom std::swap found by ADL but not used

[rkirchge at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67796

--- Comment #2 from rkirchge at gmail dot com ---
Thank you for the quick reply.

Defining the swap overload in std does violate the standard, but it is the
shortest example that reproduces the behavior I am seeing.

In my code, I have defined a wrapper around std::tuple, and defined my own swap
and get function in the associated namespace. I am trying to get std::sort to
call my swap function to allow me to swap values stored in tuples of
references. In clang my code works. In gcc if I do not include my swap(...)
definition the compiler returns an error, but if I do include it it gives no
error but uses some default std::swap implementation which does not work with
tuples of references.

My overload of get(...) is called correctly in gcc, but swap(...) is not. 

It will take me some time to cleanup my code, but I will post the example as
requested as soon as possible.