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

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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.
This commit is contained in:
Eric Botcazou 2022-04-21 00:03:59 +02:00 committed by Pierre-Marie de Rodat
parent 3e93d2926a
commit fabf93dc1a
1 changed files with 28 additions and 1 deletions
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29
gcc/ada/sem_ch6.adb
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@ -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