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

[splinterofchaos at gmail dot com]

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

--- Comment #2 from splinterofchaos at gmail dot com ---
Attempting a work-around on this problem...

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

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

  template<class = std::enable_if_t<true>>
  RETURN_TYPE operator--() & {
    return 3;
  }

  template<class = std::enable_if_t<true>>
  RETURN_TYPE operator--() const & {
    return 4;
  }
};

So, non-template functions have higher precedence, and I don't think it's valid
to convert an S& to an S&& or const S&&. But in fact, gcc (at least, 4.9) ends
up returning 1 for f3() and 2 for f4(). So, given an S&, it calls the S&&
overload, and given a const S&, it calls the const S&& overload.

clang, btw, does give the correct result, and rejects this code if you remove
the & and const & overloads.

So, this bug is maybe a bit bigger than ambiguity: gcc is picking an invalid
overload, and /that/'s creating the ambiguity!