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From: "h2+bugs at fsfe dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107202] New: inheriting assignment operators from CRTP-base
Date: Mon, 10 Oct 2022 19:32:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107202

            Bug ID: 107202
           Summary: inheriting assignment operators from CRTP-base
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: h2+bugs at fsfe dot org
  Target Milestone: ---

I stumbled over the following:


```cpp
#define OPTION 0 // or 1-4

template <typename Derived>
struct Base
{
    Base() =3D default;
    Base(Base const &) =3D default;
    Base(Base &&) =3D default;
    //Base & operator=3D(Base const &) =3D delete;
    //Base & operator=3D(Base &&) =3D delete;

#if OPTION =3D=3D 1
    Derived const & operator=3D(Derived &)
    {
        return static_cast<Derived &>(*this);
    }
#elif OPTION =3D=3D 2
    Derived const & operator=3D(Derived &) const
    {
        return static_cast<Derived &>(*this);
    }
#elif OPTION =3D=3D 3
    Derived const & operator=3D(Derived const &)
    {
        return static_cast<Derived &>(*this);
    }
#elif OPTION =3D=3D 4
    Derived const & operator=3D(Derived const &) const
    {
        return static_cast<Derived &>(*this);
    }
#endif
};

struct D : Base<D>
{
    D() =3D default;
    D(D const &) =3D default;
    D(D &&) =3D default;
    //D & operator=3D(D const &) =3D delete;
    //D & operator=3D(D &&) =3D delete;

    using Base<D>::operator=3D;
};

int main()
{
    D d1;
    D d2;
    d1 =3D d2;
}
```


OPTION =3D=3D 0 =E2=86=92 no valid overload (expected!)
OPTION =3D=3D 1 =E2=86=92 well-formed
OPTION =3D=3D 2 =E2=86=92 ambiguous overload
OPTION =3D=3D 3 =E2=86=92 no valid overload
OPTION =3D=3D 4 =E2=86=92 no valid overload

Clang behaves the same as GCC. MSVC never sees any valid overloads.

My assumption was that implicit assignment operators are inhibited in all c=
ases
(this is also shown by OPTION =3D=3D 0), but that any of the inherited oper=
ators
should be valid. Should the implicit deletion of the assignment-operator in=
 D
prevent inheriting the user-defined assignment operators from Base, why does
this affect some of them and not others?=