[gcc r16-7472] c++: relax ref-qual overloading rules for C++20 [PR98939]

[Patrick Palka]

https://gcc.gnu.org/g:ae37ade22ecf4e4ca2911d6628f8bec20c3d6422

commit r16-7472-gae37ade22ecf4e4ca2911d6628f8bec20c3d6422
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Feb 11 21:35:35 2026 -0500

    c++: relax ref-qual overloading rules for C++20 [PR98939]
    
    As explained in one of Tomasz's library papers P2438R2[1], C++20 allows
    ref-qualified member overloads to coexist with non-ref-qualified ones,
    but at the time of writing no compiler supported that.  This patch
    implements this C++20 relaxation so that we can in turn implement
    P2438R2 as originally intended -- without needing to change the
    signature of the main string::substr overload.
    
    [1]: https://wg21.link/P2438R2#_modifying_existing_const_overload
    
            PR c++/98939
            PR libstdc++/119745
    
    gcc/cp/ChangeLog:
    
            * class.cc (object_parms_correspond): Allow differing
            FUNCTION_REF_QUALIFIED in C++20.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/ref-qual5.C: Expect no diagnostics in C++20.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/class.cc                        | 7 ++++---
 gcc/testsuite/g++.dg/cpp0x/ref-qual5.C | 7 ++++---
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 315ffd316ffb..a223f71dbbe9 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -1223,9 +1223,10 @@ object_parms_correspond (tree fn, tree method, tree context)
       && DECL_IOBJ_MEMBER_FUNCTION_P (method))
     {
       /* Either both or neither need to be ref-qualified for
-	 differing quals to allow overloading.  */
-      if ((FUNCTION_REF_QUALIFIED (fn_type)
-	   == FUNCTION_REF_QUALIFIED (method_type))
+	 differing quals to allow overloading before C++20 (P1787R6).  */
+      if ((cxx_dialect >= cxx20
+	   || (FUNCTION_REF_QUALIFIED (fn_type)
+	       == FUNCTION_REF_QUALIFIED (method_type)))
 	  && (type_memfn_quals (fn_type) != type_memfn_quals (method_type)
 	      || type_memfn_rqual (fn_type) != type_memfn_rqual (method_type)))
 	return false;
diff --git a/gcc/testsuite/g++.dg/cpp0x/ref-qual5.C b/gcc/testsuite/g++.dg/cpp0x/ref-qual5.C
index e3d26e5a78fa..42e9bd3b4aee 100644
--- a/gcc/testsuite/g++.dg/cpp0x/ref-qual5.C
+++ b/gcc/testsuite/g++.dg/cpp0x/ref-qual5.C
@@ -1,5 +1,6 @@
 // 13.1: ...cannot be overloaded if any of them, but not all, have a
 // ref-qualifier.
+// In C++20 this was relaxed by P1787R6 "Declarations and where to find them"
 
 // { dg-require-effective-target c++11 }
 
@@ -7,7 +8,7 @@ class Y {
   void h() &;
   void h() const &;	       // OK
   void h() &&;		       // OK, all declarations have a ref-qualifier
-  void i() &;		       // { dg-message "" }
-  void i() const;	       // { dg-error "" } prior declaration of i
-			       // has a ref-qualifier
+  void i() &;		       // { dg-message "" "" { target c++17_down } }
+  void i() const;	       // { dg-error "" "" { target c++17_down } }
+			       // prior declaration of i has a ref-qualifier
 };