[Bug c++/78925] New: Inline friend template function not hidden during ADL

[bluescarni at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78925

            Bug ID: 78925
           Summary: Inline friend template function not hidden during ADL
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bluescarni at gmail dot com
  Target Milestone: ---

Consider the code:

--------------

namespace ns
{

template <typename T>
struct foo
{
        template <typename U>
        friend void bar(U) {}
};

}

int main()
{
        ns::foo<int> f_int;
        bar(f_int);

        ns::foo<double> f_double;
        bar(f_double);
}

--------------

Clang compiles this code without complaining, but every GCC version I have
tried (namely, 5.4.0 and the current trunk) fails, complaining about the
redefinition of ns::bar(). What seems to happen here is that during the lookup
for the second bar() call, the instantiation of bar() triggered by the first
call is considered via ADL, thus leading to two identical overloads being
present at the same time in the candidate set.