[Bug c++/104418] New: Error inheriting base class constructors by using-declaration

[Bug c++/104418] New: Error inheriting base class constructors by using-declaration

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

            Bug ID: 104418
           Summary: Error inheriting base class constructors by
                    using-declaration
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This code
```
struct B {
    B(int) {}
    B(int&&) {}
};

int i = 1;
B b(i); //ok everywhere

struct C : B {
    using B::B;
};

C c(i); //ok in Clang only
```
is accepted in Clang, but GCC complains:
```
<source>:13:6: error: use of deleted function 'C::C(int) [inherited from B]'
   13 | C c(i);
      |      ^
<source>:10:14: note: 'C::C(int) [inherited from B]' is implicitly deleted
because the default definition would be ill-formed:
   10 |     using B::B;
      |              ^
<source>:10:14: error: call of overloaded 'B(int)' is ambiguous
```
Demo: https://gcc.godbolt.org/z/79EoYM94d

Related question: https://stackoverflow.com/q/71005539/7325599

[Bug c++/104418] Error inheriting base class constructors by using-declaration

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC and ICC reject this for the same reason as GCC. Are you sure this is not a
bug in clang?

[Bug c++/104418] Error inheriting base class constructors by using-declaration

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

--- Comment #2 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
My personal feeling is that if a compiler accepts `B b(i);` then it must accept
`C c(i);` as well because of [namespace.udecl] p13:

> Constructors that are named by a using-declaration are treated as though they were constructors of the derived class when looking up the constructors of the derived class ([class.qual]) or forming a set of overload candidates ([over.match.ctor], [over.match.copy], [over.match.list]).

But probably I miss something.

[Bug c++/104418] Error inheriting base class constructors by using-declaration

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
But this is invalid code with a similar error message:
struct B {
    B(int) {}
    B(int&&) {}
};
struct C : B {
    C(int a) : B{(int)a}{}
};

So the question is how is default constructor done, is it done with the cast
causing an rvalue or not?

[Bug c++/104418] Error inheriting base class constructors by using-declaration

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

--- Comment #4 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
I think `using B::B;` is not the same as redefining each constructor with the
explicit call of base class constructor `C(int a) : B{(int)a}{}`.

Please consider this example proving it:
```
struct A {
    A() {}
    A(const A&) = delete;
};

struct B {
    B(A) {}
};

// ok everywhere
struct C : B {
    using B::B;
};

//error everywhere
struct D : B {
    D(A a) : B(a) {}
};
```

Here struct C definition is fine, but struct D is ill-formed because it
requires A to be copyable. Demo: https://gcc.godbolt.org/z/T7bhv9jeP

[Bug c++/104418] [C++14+] Error inheriting base class constructors by using-declaration

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
            Summary|Error inheriting base class |[C++14+] Error inheriting
                   |constructors by             |base class constructors by
                   |using-declaration           |using-declaration
   Last reconfirmed|                            |2022-02-08
           Keywords|                            |rejects-valid

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Fedor Chelnokov from comment #4)
> I think `using B::B;` is not the same as redefining each constructor with
> the explicit call of base class constructor `C(int a) : B{(int)a}{}`.

Sorry I meant it should be done by static_cast<int&&>(a) which is the move
syntax rather than by a copy.


There looks like there are some differences between versions of the C++
standard too.
C++11 (before defect reports) In class.inhctor/8 had:
Each expression in the expression-list is of the form static_cast<T&&>(p),
where p is the name
of the corresponding constructor parameter and T is the declared type of p.

But that was removed it looks like with
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0136r1.html 

So the code is invalid C++11 but valid C++14 it seems. clang gets it wrong for
C++11 but gets it correct for C++14+.

GCC gets it wrong for C++14+.

[Bug c++/104418] [C++17+] Error inheriting base class constructors by using-declaration

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[C++14+] Error inheriting   |[C++17+] Error inheriting
                   |base class constructors by  |base class constructors by
                   |using-declaration           |using-declaration

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #5)
> So the code is invalid C++11 but valid C++14 it seems. clang gets it wrong
> for C++11 but gets it correct for C++14+.
> 
> GCC gets it wrong for C++14+.

Or that is part of C++17. Someone else will have to decide there. But clang is
definitely wrong for C++11 and GCC is definitely wrong for C++17 and C++20.

[Bug c++/104418] [C++17+] Error inheriting base class constructors by using-declaration

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104418

--- Comment #7 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
Thanks. I submitted Clang bug:
https://github.com/llvm/llvm-project/issues/53653