[Bug c++/64562] New: Member functions with r-value reference for *this and deduced return type incorrectly rejected as ambiguous

[sneves at dei dot uc.pt]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64562

            Bug ID: 64562
           Summary: Member functions with r-value reference for *this and
                    deduced return type incorrectly rejected as ambiguous
           Product: gcc
           Version: 4.9.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sneves at dei dot uc.pt

Consider the following program:

#include <iostream>

#define RETURN_TYPE auto

namespace {

struct S {
  RETURN_TYPE operator--() && {
    return 1;
  }

  RETURN_TYPE operator--() const && {
    return 2;
  }

  RETURN_TYPE operator--() & {
    return 3;
  }

  RETURN_TYPE operator--() const & {
    return 4;
  }
};

const S callme() { return {}; }

auto f1() {
  return --S();
}

auto f2() {
  return --callme();
}

auto f3() {
  S s;
  return --s;
}

auto f4() {
  const S s {};
  return --s;
}

}

int main() {
  std::cout << f1() << " ";
  std::cout << f2() << " ";
  std::cout << f3() << " ";
  std::cout << f4() << std::endl;
}

The expected output would be "1 2 3 4". When RETURN_TYPE is defined to be
"int", this is the result obtained. GCC (versions ranging from 4.8.2 to the
latest build), however, rejects this code when RETURN_TYPE is defined to be
"auto" or "decltype(auto)".