[Bug c++/107889] New: Incorrect parsing of qualified friend function returning decltype(auto)

[Bug c++/107889] New: Incorrect parsing of qualified friend function returning decltype(auto)

[gcc at nospam dot scs.stanford.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107889

            Bug ID: 107889
           Summary: Incorrect parsing of qualified friend function
                    returning decltype(auto)
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at nospam dot scs.stanford.edu
  Target Milestone: ---

G++ 12.2.0 rejects a valid friend declaration for a fully-qualified function
returning `decltype(auto)`.  To reproduce the problem, you can try to compile
the following code with `g++ -std=c++20 -c bug.cc`:

decltype(auto)
f()
{
}

struct S {
  friend decltype(auto) ::f();
};


This results in the following error:

$ c++ -std=c++20 -c bug.cc
bug.cc:7:27: error: 'decltype(auto)' is not a class type
    7 |   friend decltype(auto) ::f();
      |                           ^
bug.cc:7:27: error: 'decltype(auto)' is not a class type
bug.cc:7:27: error: 'decltype(auto)' is not a class type
bug.cc:7:29: error: 'decltype(auto)' is not a class type
    7 |   friend decltype(auto) ::f();
      |                             ^
bug.cc:7:10: error: ISO C++ forbids declaration of 'f' with no type
[-fpermissive]
    7 |   friend decltype(auto) ::f();
      |          ^~~~~~~~~~~~~~
bug.cc:7:29: error: invalid use of 'decltype(auto)'
    7 |   friend decltype(auto) ::f();


A similar problem was reported in bug #59766 for friend functions returning
auto.  It seems to have been mostly fixed, but the combination of
decltype(auto) and the function name being qualified (::f) is still a problem.

[Bug c++/107889] Incorrect parsing of qualified friend function returning decltype(auto)

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107889

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-11-28
                 CC|                            |marxin at gcc dot gnu.org

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
Clang accepts the code.

[Bug c++/107889] Incorrect parsing of qualified friend function returning decltype(auto)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107889

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think this is https://wg21.link/cwg1828 which was resolved by
https://wg21.link/p1787 which GCC does not implement yet.

[Bug c++/107889] Incorrect parsing of qualified friend function returning decltype(auto)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107889

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to David Mazières from comment #0)
> A similar problem was reported in bug #59766 for friend functions returning
> auto.

That's unrelated.

The problem here is that decltype(auto)::f is a nested-name-specifier for a
member 'f' so is parsed as part of the return type, not the function name. This
is what C++20 (and all earlier standards) said. The ambiguity has been resolved
as a DR against earlier standards, but GCC doesn't implement that change yet,
as shown at https://gcc.gnu.org/projects/cxx-status.html#cxx23