[Ada] Unnesting: fix handling of uplevel refs to unconstrained formals

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Unnest_Subprogram):  When an uplevel reference
	is to an unconstrained formal, the 'Access reference that is
	created to initialize the corresponding component of the
	activation record must be wrapped in an unchecked conversion to
	the generated type of the component. Otherwise, spurious suvtype
	conformance errors will be generated when the code is within an
	instantiation and the type of the formal is a formal type of the
	enclosing generic. Note that during unnesting there is no simple
	way to determine that the code appears within an instance
	because ther is no scope stack.

From-SVN: r264972

gcc/ada/ChangeLog

+2018-10-09  Ed Schonberg  <schonberg@adacore.com>
+
+	* exp_unst.adb (Unnest_Subprogram):  When an uplevel reference
+	is to an unconstrained formal, the 'Access reference that is
+	created to initialize the corresponding component of the
+	activation record must be wrapped in an unchecked conversion to
+	the generated type of the component. Otherwise, spurious suvtype
+	conformance errors will be generated when the code is within an
+	instantiation and the type of the formal is a formal type of the
+	enclosing generic. Note that during unnesting there is no simple
+	way to determine that the code appears within an instance
+	because ther is no scope stack.
+
 2018-10-09  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gcc-interface/decl.c (type_requires_init_of_formal): New

gcc/ada/exp_unst.adb

@@ -1966,7 +1966,9 @@ package body Exp_Unst is
 
                                  Asn  : Node_Id;
                                  Attr : Name_Id;
+                                 Comp : Entity_Id;
                                  Ins  : Node_Id;
+                                 Rhs  : Node_Id;
 
                               begin
                                  --  For parameters, we insert the assignment
@@ -2001,6 +2003,32 @@ package body Exp_Unst is
                                     Attr := Name_Address;
                                  end if;
 
+                                 Rhs :=  Make_Attribute_Reference (Loc,
+                                         Prefix         =>
+                                           New_Occurrence_Of (Ent, Loc),
+                                         Attribute_Name => Attr);
+
+                                 --  If the entity is an unconstrained formal
+                                 --  we wrap the attribute reference in an
+                                 --  unchecked conversion to the type of the
+                                 --  activation record component, to prevent
+                                 --  spurious subtype conformance errors within
+                                 --  instances.
+
+                                 if Is_Formal (Ent)
+                                   and then not Is_Constrained (Etype (Ent))
+                                 then
+                                    --  Find target component and its type.
+
+                                    Comp := First_Component (STJ.ARECnT);
+                                    while Chars (Comp) /= Chars (Ent) loop
+                                       Comp := Next_Component (Comp);
+                                    end loop;
+
+                                    Rhs := Unchecked_Convert_To (
+                                              Etype (Comp), Rhs);
+                                 end if;
+
                                  Asn :=
                                    Make_Assignment_Statement (Loc,
                                      Name       =>
@@ -2012,12 +2040,7 @@ package body Exp_Unst is
                                              (Activation_Record_Component
                                                 (Ent),
                                               Loc)),
-
-                                     Expression =>
-                                       Make_Attribute_Reference (Loc,
-                                         Prefix         =>
-                                           New_Occurrence_Of (Ent, Loc),
-                                         Attribute_Name => Attr));
+                                     Expression => Rhs);
 
                                  --  If we have a loop parameter, we have
                                  --  to insert before the first statement