[Bug c++/100465] Overloading operator+= and including filesystem causes conflicting overload compilation error

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100465

--- Comment #7 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #6)
> (In reply to Jonathan Wakely from comment #2)
> > Maybe another case of PR 51577 but I haven't looked into it yet.
> 
> The testcase in comment 4 was fixed by the patch for that bug, r12-702.
> 
> The original testcase using <filesystem> still fails though.
> 
> Patrick, do you think this is just a dup of PR 51577? Do I need to reduce
> this again to something that still fails, or do we have a matching testcase
> already?

r12-702 should have fixed operator lookup in the block scope case, so I'd
expect the original <filesystem> testcase to work since the problematic name
lookup occurs at block scope.  I think the other unresolved testcases
associated with PR51577 all have to do with name lookup at non-block scope, so
this seems to be a distinct bug.

Here's a reduced rejects-valid testcase:

namespace N
{
  struct string
  {
    template<typename T>
    void operator+=(T);
  };

  struct A {
    void operator+=(char); // #1

    template<typename T>
    void f() {
      string s;
      s += T();
    }

    void g() {
      f<char>();
    }
  };
} // namespace N

template<typename T>
void operator+=(N::string, T);

If we comment out the line #1 then it works.