[Bug c++/86577] New: non-ADL name lookup for operator<< at instantiation time?

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86577

            Bug ID: 86577
           Summary: non-ADL name lookup for operator<< at instantiation
                    time?
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

The code is as follow:

#include <iostream>

 namespace N {
 struct X { };
 }
 using namespace N;

 template<typename T>
 void log(const T& t) {
 std::clog << t;
 } 

 std::ostream &operator<<(std::ostream&, const X& x);

 template void log(const X&);


g++ accepts the code, but clang++ rejects it:

code0.cpp:10:12: error: call to function 'operator<<' that is neither visible
      in the template definition nor found by argument-dependent lookup
 std::clog << t;
           ^
code0.cpp:15:16: note: in instantiation of function template specialization
      'log<N::X>' requested here
 template void log(const X&);
               ^
code0.cpp:13:16: note: 'operator<<' should be declared prior to the call site
      or in namespace 'N'
 std::ostream &operator<<(std::ostream&, const X& x);
               ^
1 error generated.

Does g++ perform non-ADL name lookup for operator<< at instantiation time? This
sounds incorrect.