[Bug c++/98410] New: Default constructor generation fails on abstract class with virtual base

["thelordlucas at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98410

            Bug ID: 98410
           Summary: Default constructor generation fails on abstract class
                    with virtual base
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thelordlucas at gmail dot com
  Target Milestone: ---

Hi all,

The following code fails to compile on GCC > 7:

class Base {
    int x;
public:
    Base(int x): x{x} {}
    virtual void f() = 0;
};

class Derived : public virtual Base  {
  public:
    Derived() = default;
};

class Concrete: public Derived {
public:
    Concrete(): Base{42} {}
    void f() override {}
};

Link to code: https://godbolt.org/z/bn1EY6


It says that the Derived() constructor is deleted (error: use of deleted
function 'Derived::Derived()'). Using Derived() {}; it compiles file. 

As the user eerorika points out in
https://stackoverflow.com/questions/65287323/strange-default-empty-constructor-on-a-virtual-inheritance-behaviour-on-gcc
the constructor Derived() should not have been deleted, since it is also
abstract and doesn't fall in any default constructor deletion rule.

Maybe related, it yielded "internal compiler error: in
maybe_explain_implicit_delete, at cp/method.c:2671" if I add the noexcept to
the deleted constructor:

class Base {
    int x;
public:
    Base(int x): x{x} {}
    virtual void f() = 0;
};

class Derived : public virtual Base  {
  public:
    Derived() noexcept = default;
};

class Concrete: public Derived {
public:
    Concrete(): Base{42} {}
    void f() override {}
};

Link to code: https://godbolt.org/z/5fx8Ph

I tested clang trunk, and it compiles both versions.

Thanks!