[Bug c++/96647] New: Can't resolve pointer to overloaded member with auto return type

[Bug c++/96647] New: Can't resolve pointer to overloaded member with auto return type

[rustamabd at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96647

            Bug ID: 96647
           Summary: Can't resolve pointer to overloaded member with auto
                    return type
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rustamabd at gmail dot com
  Target Milestone: ---

auto return type in combination with overloading and CRTP results in an
overload set that cannot be disambiguated.

Example:

template<typename T>
struct Base {
    auto f(int) { return static_cast<T*>(this); }
    auto f(char) { return static_cast<T*>(this); }
};

struct A : Base<A> {};

int main() {
    A instance;

    // instance.f('x') // "fixes" the issue

    auto ptr = static_cast<A* (A::*)(char)>(&A::f); // error here
    (instance.*ptr)('x');
}

Error:

error: no matches converting function 'f' to type 'struct A* (struct
A::*)(char)'
   14 |     auto ptr = static_cast<A* (A::*)(char)>(&A::f); // error here
      |                                                  ^
note: candidates are: 'auto Base<T>::f(char) [with T = A]'
    4 |     auto f(char) { return static_cast<T*>(this); }
      |          ^
note:                 'auto Base<T>::f(int) [with T = A]'
    3 |     auto f(int) { return static_cast<T*>(this); }
      |          ^

Replacing "auto" with "T*", as well as invoking instance.f('x'), fixes the
issue.

clang compiles the code fine.

[Bug c++/96647] Can't resolve pointer to overloaded member with auto return type

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96647

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Target Milestone|---                         |11.0
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2020-08-17
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, not a regression.

[Bug c++/96647] Can't resolve pointer to overloaded member with auto return type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96647

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:4e03e2e08b94f65f2be3db17f880c205ec262f87

commit r11-3075-g4e03e2e08b94f65f2be3db17f880c205ec262f87
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Sep 9 09:21:09 2020 -0400

    c++: Fix resolving the address of overloaded pmf [PR96647]

    In resolve_address_of_overloaded_function, currently only the second
    pass over the overload set (which considers just the function templates
    in the overload set) checks constraints and performs return type
    deduction when necessary.  But as the testcases below show, we need to
    do the same when considering non-template functions during the first
    pass.

    gcc/cp/ChangeLog:

            PR c++/96647
            * class.c (resolve_address_of_overloaded_function): Check
            constraints_satisfied_p and perform return-type deduction via
            maybe_instantiate_decl when considering non-template functions
            in the overload set.
            * cp-tree.h (maybe_instantiate_decl): Declare.
            * decl2.c (maybe_instantiate_decl): Remove static.

    gcc/testsuite/ChangeLog:

            PR c++/96647
            * g++.dg/cpp0x/auto-96647.C: New test.
            * g++.dg/cpp0x/error9.C: New test.
            * g++.dg/cpp2a/concepts-fn6.C: New test.

[Bug c++/96647] Can't resolve pointer to overloaded member with auto return type

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96647

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.  Thanks for the report!