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From: "ammiera at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/101698] New: Template type conversion operator from base
 class preferred over matching overload
Date: Fri, 30 Jul 2021 19:11:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101698

            Bug ID: 101698
           Summary: Template type conversion operator from base class
                    preferred over matching overload
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ammiera at hotmail dot com
  Target Milestone: ---

There are two conversion operators, template and non-templated.
Their return type might match:

virtual operator const std::string&() const;
template <class T> operator const T&() const;

If so, when trying to call it, the template is always preferred over
non-template and there's no way to SFINAE out of it:


#include <iostream>
#include <string>
#include <type_traits>

class Base {
 public:
  template <class T>
  operator const T&() const {
    std::cout << "use template method" << std::endl;
    static T tmp{};
    return tmp;
  }

  virtual operator const std::string&() const {
    std::cout << "use overload method" << std::endl;
    const static std::string tmp;
    return tmp;
  }
};

template <class T>
class Derive : public Base {
 public:
  operator const T&() const override {
    using Y =3D std::string;
    static_assert(std::is_same<T, Y>::value, "");

    std::string static res;

    res =3D Base::operator const Y&();
    res =3D Base::operator const T&();
    return res;
  }
};

int main() {
  Derive<std::string> a;
  const std::string& b =3D a;
  (void)b;
}

Reproduced locally on:
gcc (SUSE Linux) 11.1.1 20210721 [revision
076930b9690ac3564638636f6b13bbb6bc608aea]
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Did a quick bisection via godbolt.org,  the last version it's didn't occur =
is
7.5.

https://gcc.godbolt.org/z/KcbK9vv5z=