C++23: using-declaration naming a member hidden in every direct base is still rejected (P1787R6)

[the4amfriend]

Issue Summary

P1787R6 (adopted for C++23) removed the [namespace.udecl] requirement that the base class members named by a using-declaration "shall be visible in the scope of at least one of the direct base classes". EDG still applies the requirement in --c++23 and --c++26 modes, so it rejects valid C++23 code.

Under C++17 and C++20 the rejection matches the standard text. This report is only about C++23 and later modes.

Reproducing Source Code

struct A {
  virtual ~A() {}
  virtual bool f(int) const { return false; }
  virtual bool f(double) const { return false; }
};
struct B : A {
  using A::f;
  bool f(int) const override { return false; }  // hides A::f(int) in B
};
struct C : B {
  using A::f;  // A::f(int) is not visible in B, C's only direct base
  bool f(int) const override { return true; }
};
int main() {
  C c;
  return c.f(1.0) ? 1 : 0;
}

Command-line Options

cpfe --c++23 --no_code_gen ui_twice.cc (also --c++23 --strict, --c++26)

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

Actual

"ui_twice.cc", line 14: error: class member designated by a using-declaration
          must be visible in a direct base class
    using A::f;
          ^

Expected

Accepted, and the program returns 0 (c.f(1.0) calls A::f(double)). GCC 15.3 and Clang 21.1 accept it in all modes,
-pedantic-errors included.

Notes

• The check is check_member_using_visibility() in src/class_decl.c, called only for indirect bases, and already skipped in cfront, g++/Clang and MSVC > 12.00 modes.
• EDGcpfe/28870 (src/Changes, 6/25/26) gated another P1787R6 relaxation on cpp23_mode. The same approach fits here.
• I have a small patch with regression tests ready and can open a PR once there is an issue number.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
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