[Bug c++/88725] New: fails to deduce template specialization in friend declaration

[haining.cpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88725

            Bug ID: 88725
           Summary: fails to deduce template specialization in friend
                    declaration
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haining.cpp at gmail dot com
  Target Milestone: ---

gcc fails to deduce that the friend declaration refers to ::func<int> in the
below

```
template <typename T>
void func(T);

class Cls {
    friend void ::func(int);
};
```

Rejecting with 

```
error: 'void func(int)' should have been declared inside '::'
     friend void ::func(int);
                           ^
```

I believe this should be allowed by [temp.friend]/1.3:
http://eel.is/c++draft/temp.friend#1.3

see discussion here: https://stackoverflow.com/q/54055575/1013719