[Bug c++/68942] New: overly strict use of deleted function before argument-dependent lookup (ADL)

[lucdanton at free dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68942

            Bug ID: 68942
           Summary: overly strict use of deleted function before
                    argument-dependent lookup (ADL)
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lucdanton at free dot fr
  Target Milestone: ---

Tested on 5.2.0, 5.2.1 and 6.

I believe the testcase is incorrectly rejected. Clang accepts the program. Note
that leaving void foo(int); undefined, rather than defined as deleted, lets GCC
accept the program.

$ g++-trunk -std=c++14 main.cpp
main.cpp: In function 'void lookup(X)':
main.cpp:5:3: error: use of deleted function 'void foo(int)'
 { foo(x); }
   ^~~

main.cpp:1:6: note: declared here
 void foo(int) = delete;
      ^~~

$ cat main.cpp
void foo(int) = delete;

template<typename X>
void lookup(X x)
{ foo(x); }

namespace ns {

struct dummy {};
int foo(dummy) { return 3; }

} // ns

int main()
{
    lookup(ns::dummy {});
}