[Bug c++/92590] New: Cannot expose protected default constructor with "using" keyword in gcc 10

[proski at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92590

            Bug ID: 92590
           Summary: Cannot expose protected default constructor with
                    "using" keyword in gcc 10
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: proski at gnu dot org
  Target Milestone: ---

This code fails to compile with the current gcc (commit
d9be9f34fbb018c448dc5a02aaa95d6a6932135c)


class Base {
  protected:
    Base();
};

class Derived : public Base {
  public:
    using Base::Base;
};

Derived d;


test.ii:11:9: error: ‘Base::Base()’ is protected within this context
   11 | Derived d;
      |         ^
test.ii:3:5: note: declared protected here
    3 |     Base();
      |     ^~~~

The error is emitted for any standard from c++11 to c++2a. No optimization or
other flags are needed.

The same code is compiled without any errors by gcc 9.2.1. Testing on
godbolt.org shows that all other compilers accept it - all recent versions of
clang, msvc, icc.

I don't see any relevant changes mentioned in the GCC 10 release notes
https://gcc.gnu.org/gcc-10/changes.html

An inline implementation of the protected constructor fails too:


class Base {
  protected:
    Base() {}
};

class Derived : public Base {
  public:
    using Base::Base;
};

Derived d;


However, the code compiles with the default constructor:


class Base {
  protected:
    Base() = default;
};

class Derived : public Base {
  public:
    using Base::Base;
};

Derived d;