[gcc r13-829] [Ada] Restore hiding of predefined "=" operator through class-wide type

[Pierre-Marie de Rodat <pmderodat@gcc.gnu.org>]

https://gcc.gnu.org/g:fabf93dc1ae8268c416c1f2b77425922a6de046f

commit r13-829-gfabf93dc1ae8268c416c1f2b77425922a6de046f
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Thu Apr 21 00:03:59 2022 +0200

    [Ada] Restore hiding of predefined "=" operator through class-wide type
    
    The previous change introduced a backward incompatibility in the handling
    of a user-defined "=" operator for a class-wide type of a tagged type: it
    would previously hide the predefined "=" operator of the tagged type in
    the private case, but it no longer does in this case, while it still does
    in the nonprivate case.
    
    This hiding is a non-portability, but is fundamentally what the compiler
    implements, instead of the RM rule which requires homographs.  The reason
    lies in the implementation of the "=" operator in GNAT: internally, there
    is not a "=" predefined operator for every nonlimited type, but instead
    there is a single, universal "=" predefined operator for all the nonlimited
    types.  The consequence is that the hiding rule implemented in GNAT for "="
    is effectively that a user-declared symmetrical "=" operator returning
    boolean hides the predefined "=" operator of any type that is covered by
    the user-declared operator.
    
    Whether it is desirable to implement the exact RM rule in GNAT is to be
    discussed, but existing code relies on the non-portability and would thus
    need to be changed.
    
    gcc/ada/
    
            * sem_ch6.adb (New_Overloaded_Entity): Deal specifically with the
            overriding of the "=" operator for tagged types.

Diff:
---
 gcc/ada/sem_ch6.adb | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
index a5373583207..cb2ed7c5e45 100644
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -12303,7 +12303,34 @@ package body Sem_Ch6 is
                            --  Work done in Override_Dispatching_Operation, so
                            --  nothing else needs to be done here.
 
-                           null;
+                           --  ??? Special case to keep supporting the hiding
+                           --  of the predefined "=" operator for a nonlimited
+                           --  tagged type by a user-defined "=" operator for
+                           --  its class-wide type when the type is private.
+
+                           if Chars (E) = Name_Op_Eq then
+                              declare
+                                 Typ : constant Entity_Id
+                                         := Etype (First_Entity (E));
+                                 H   : Entity_Id := Homonym (E);
+
+                              begin
+                                 while Present (H)
+                                   and then Scope (H) = Scope (E)
+                                 loop
+                                    if Is_User_Defined_Equality (H)
+                                       and then Is_Immediately_Visible (H)
+                                       and then Etype (First_Entity (H))
+                                                  = Class_Wide_Type (Typ)
+                                    then
+                                       Remove_Entity_And_Homonym (E);
+                                       exit;
+                                    end if;
+
+                                    H := Homonym (H);
+                                 end loop;
+                              end;
+                           end if;
                         end if;
 
                      else