[Bug c++/100335] New: Using statement of a ref-qualified method from base class: method not callable on derived object

[Daniel.Withopf at web dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100335

Bug ID: 100335
   Summary: Using statement of a ref-qualified method from base
class: method not callable on derived object
   Product: gcc
   Version: 10.3.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: Daniel.Withopf at web dot de
  Target Milestone: ---

The following code examples defines 2 ref-qualified method with identical names
in a base class (const& and const&&). Derived also defines a (non-const) method
with the same name and adds a "using Base::method;"

With all gcc Versions I tested (10.3, 9.3, 8.3, 7.5), the compilation with
--std=c++11 or --std=c++14 fails if the method in the derived class does not
have a ref-qualifier.

class Base {
public:
  void method() const&& {}
  void method() const& {}
};

class Derived : public Base {
public:
  using Base::method;
  // this leads to a compiler error
  void method() {}

  // with a ref-qualifier the code is compiling
  //  void method() & {}
};

int main() {
const Derived test;
test.method();
}

I believe that this is incorrect and the code should also compile when the
method in the Derived class has no ref-qualifier.

[Bug c++/100335] Using statement of a ref-qualified method from base class: method not callable on derived object

[Daniel.Withopf at web dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100335

--- Comment #2 from Daniel  ---
As an extra Info: the other compilers I tested (e.g. clang) accept the code
example as is.

But after reading the cited pet of the standard It seems that GCC is right in
rejecting this and the other compilers have an issue.

[Bug c++/100335] Using statement of a ref-qualified method from base class: method not callable on derived object

[Daniel.Withopf at web dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100335

--- Comment #5 from Daniel  ---
As a sidenote, the original example is also compiling if test object is made
non-const, i.e. "const Derived test;" is replaced with "Derived test;"

If the argument in Comment 1 is true than the program would still be ill-formed
in this case, wouldn't it?

[Bug c++/100335] Using statement of a ref-qualified method from base class: method not callable on derived object

[Daniel.Withopf at web dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100335

--- Comment #7 from Daniel  ---
To me it seems that [over.load] is the right section of the standard as the
start of the section explicitly mentions that the rules there (either all or
none of the overloads must have ref-qualifiers) applies when a using
declaration is involved:

"Not all function declarations can be overloaded. Those that cannot be
overloaded are specified here. A program is ill-formed if it contains two such
non-overloadable declarations in the same scope. [Note:This restriction applies
to explicit declarations in a scope, and between such declarations and
declarations made through a using-declaration(7.3.3). It does not apply to sets
of functions fabricated as a result of name lookup (e.g., because of
using-directives) or overload resolution (e.g., for operator functions).— end
note]"

Could it be that
- gcc is on the right track by rejecting the example, but it should also reject
the code in the case where the offending method is not called (see Comment 6)?
- clang, MSVC are not taking the above note into consideration and thus are
missing an error in this case?