ada: Further fixes to handling of private views in instances

This removes more bypasses for private views in instances that are present
in type predicates (Conforming_Types, Covers, Specific_Type and Wrong_Type),
which in exchange requires additional work in Sem_Ch12 to restore the proper
view of types during the instantiation of generic bodies.

The main mechanism for this is the Has_Private_View flag, but it comes with
the limitations that 1) there must be a direct reference to the global type
in the generic construct (either a reference to a global object of this type
or the explicit declaration of a local object of this type), which is not
always the case e.g. for loop parameters and 2) it can deal with a single
type at a time, e.g. it cannot deal with an array type and its component
type if their respective views are not the same in the instance.

To overcome the second limitation, a new Has_Secondary_Private_View flag
is introduced to deal with a secondary type, which as of this writing is
either the component type of an array type or the designated type of an
access type (together they make up the vast majority of the problematic
cases for the Has_Private_View flag alone). This new mechanism subsumes
a specific treatment for them that was added in Copy_Generic_Node a few
years ago, although a specific treatment still needs to be preserved for
comparison and equality operators in a narrower case.

Additional handling is also introduced to overcome the first limitation
for loop parameters in Copy_Generic_Node, and a relaxed condition is used
in Exp_Ch7.Convert_View to generate an unchecked conversion between views.

gcc/ada/

	* exp_ch7.adb (Convert_View): Detect more cases of mismatches for
	private types and use Implementation_Base_Type as main criterion.
	* gen_il-fields.ads (Opt_Field_Enum): Add
	Has_Secondary_Private_View
	* gen_il-gen-gen_nodes.adb (N_Expanded_Name): Likewise.
	(N_Direct_Name): Likewise.
	(N_Op): Likewise.
	* sem_ch12.ads (Check_Private_View): Document the usage of second
	flag Has_Secondary_Private_View.
	* sem_ch12.adb (Get_Associated_Entity): New function to retrieve
	the ultimate associated entity, if any.
	(Check_Private_View): Implement Has_Secondary_Private_View
	support.
	(Copy_Generic_Node): Remove specific treatment for Component_Type
	of an array type and Designated_Type of an access type. Add
	specific treatment for comparison and equality operators, as well
	as iterator and loop parameter specifications.
	(Instantiate_Type): Implement Has_Secondary_Private_View support.
	(Requires_Delayed_Save): Call Get_Associated_Entity.
	(Set_Global_Type): Implement Has_Secondary_Private_View support.
	* sem_ch6.adb (Conforming_Types): Remove bypass for private views
	in instances.
	* sem_type.adb (Covers): Return true if Is_Subtype_Of does so.
	Remove bypass for private views in instances.
	(Specific_Type): Likewise.
	* sem_util.adb (Wrong_Type): Likewise.
	* sinfo.ads (Has_Secondary_Private_View): Document new flag.

1 files changed, 5 insertions, 12 deletions

diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
index 17c50f6..62ca985 100644
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -8410,21 +8410,14 @@ package body Sem_Ch6 is
            Ctype <= Mode_Conformant
              or else Subtypes_Statically_Match (Type_1, Full_View (Type_2));
 
-      elsif Is_Private_Type (Type_2)
-        and then In_Instance
-        and then Present (Full_View (Type_2))
-        and then Base_Types_Match (Type_1, Full_View (Type_2))
-      then
-         return
-           Ctype <= Mode_Conformant
-             or else Subtypes_Statically_Match (Type_1, Full_View (Type_2));
-
-      --  Another confusion between views in a nested instance with an
-      --  actual private type whose full view is not in scope.
+      --  The subtype declared for the formal type in an instantiation and the
+      --  actual type are conforming. Note that testing Is_Generic_Actual_Type
+      --  here is not sufficient because the flag is only set in the bodies of
+      --  instances, which is too late for formal subprograms.
 
       elsif Ekind (Type_2) = E_Private_Subtype
-        and then In_Instance
         and then Etype (Type_2) = Type_1
+        and then Present (Generic_Parent_Type (Declaration_Node (Type_2)))
       then
          return True;