[Ada] Spurious error on instantiation and limited_with_clause

This patch fixes a spurious error during the construction of an instance
body in the inlining phase of the frontend, when the package declaration
for the main unit has a limited_with_clause on some unit P, and the main
unit itself does not have a corresponding regular with_clause, but some
other unit in the context has with_clause that has compiled P.  P must
be removed from visibility to prevent conflicts with homonyms in the
generic body to be instantiated.

The following must compile quietly:

   gcc -c gpr2-project-view.adb

----
package GNATCOLL is
end GNATCOLL;
package GNATCOLL.Refcount is

   generic
      type Element_Type (<>) is private;
   package Shared_Pointers is
      type Ref is tagged private;

      type Element_Access is access all Element_Type;

      type Reference_Type (Element : access Element_Type)
         is limited null record;

      function Unchecked_Get (Self : Ref'Class) return Element_Access;

      function Get (Self : Ref'Class) return Reference_Type
         is ((Element => Unchecked_Get (Self)));
   private
      type Ref is tagged null record;
   end Shared_Pointers;

   type Refcounted is abstract tagged null record;

   generic
      type Encapsulated is abstract new Refcounted with private;
   package Smart_Pointers is
      type Encapsulated_Access is access all Encapsulated'Class;

      type Ref is tagged private;

      procedure Set (Self : in out Ref; Data : Encapsulated'Class);
      procedure Set (Self : in out Ref; Data : access Encapsulated'Class);
   private
      type Ref is tagged null record;
   end Smart_Pointers;

end GNATCOLL.Refcount;
----
package body GNATCOLL.Refcount is

   package body Shared_Pointers is

      function Unchecked_Get (Self : Ref'Class) return Element_Access is
      begin
         return null;
      end Unchecked_Get;

   end Shared_Pointers;

   package body Smart_Pointers is

      procedure Set (Self : in out Ref; Data : access Encapsulated'Class) is
      begin
         null;
      end Set;

      procedure Set (Self : in out Ref; Data : Encapsulated'Class) is
         Tmp : constant Encapsulated_Access := new Encapsulated'Class'(Data);
      begin
         Set (Self, Tmp);
      end Set;

   end Smart_Pointers;
end GNATCOLL.Refcount;
----
package GPR2 is
end GPR2;
----
package GPR2.Parser is
end GPR2.Parser;
----
with GPR_Parser.Analysis;
package GPR2.Parser.Project is
end GPR2.Parser.Project;
----
package GPR2.Project is
end GPR2.Project;
----
with GPR2.Parser.Project;
package GPR2.Project.Configuration is
end GPR2.Project.Configuration;
----
with GPR2.Project.Configuration;
with GPR2.Unit.Set;
package GPR2.Project.Definition is
end GPR2.Project.Definition;
----
limited with GPR2.Unit.Set;
package GPR2.Project.View is
   procedure Require_Body;
end GPR2.Project.View;
----
with GPR2.Project.Definition;
package body GPR2.Project.View is
   procedure Require_Body is null;
end GPR2.Project.View;
----
package GPR2.Unit is
end GPR2.Unit;

package GPR2.Unit.Set is
end GPR2.Unit.Set;
...
package GPR_Parser is
end GPR_Parser;
----
with GNATCOLL.Refcount;
package GPR_Parser.Analysis is

   type Unit_Provider_Interface is null record;

   package Unit_Provider_References is new GNATCOLL.Refcount.Shared_Pointers
     (Unit_Provider_Interface);

end GPR_Parser.Analysis;

2019-07-04  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch10.adb (Remove_Context_Clauses): Handle properly the
	removal of a limited_with_clause which appears in the library
	unit oF the main unit, when some other unit in the context has a
	regular with_clause on the same unit, to prevent spurious
	visibility errors in the subsequent analysis of pending instance
	bodies.

From-SVN: r273067

gcc/ada/ChangeLog

+2019-07-04  Ed Schonberg  <schonberg@adacore.com>
+
+	* sem_ch10.adb (Remove_Context_Clauses): Handle properly the
+	removal of a limited_with_clause which appears in the library
+	unit oF the main unit, when some other unit in the context has a
+	regular with_clause on the same unit, to prevent spurious
+	visibility errors in the subsequent analysis of pending instance
+	bodies.
+
 2019-07-04  Hristian Kirtchev  <kirtchev@adacore.com>
 
 	* sem_elab.adb: Add new type Elaboration_Phase_Status along with

gcc/ada/sem_ch10.adb

@@ -6379,22 +6379,38 @@ package body Sem_Ch10 is
 
    begin
       --  Ada 2005 (AI-50217): We remove the context clauses in two phases:
-      --  limited-views first and regular-views later (to maintain the
-      --  stack model).
+      --  limited-views first and regular-views later (to maintain the stack
+      --  model).
 
       --  First Phase: Remove limited_with context clauses
 
       Item := First (Context_Items (N));
       while Present (Item) loop
 
-         --  We are interested only in with clauses which got installed
-         --  on entry.
+         --  We are interested only in with clauses that got installed on entry
 
          if Nkind (Item) = N_With_Clause
            and then Limited_Present (Item)
-           and then Limited_View_Installed (Item)
          then
-            Remove_Limited_With_Clause (Item);
+            if Limited_View_Installed (Item) then
+               Remove_Limited_With_Clause (Item);
+
+            --  An unusual case: If the library unit of the Main_Unit has
+            --  a limited with_clause on some unit P and the context somewhere
+            --  includes a with_clause on P, P has been analyzed. The entity
+            --  for P is still visible, which in general is harmless because
+            --  this is the end of the compilation, but it can affect pending
+            --  instantiations that may have been generated elsewhere, so it
+            --  it is necessary to remove U from visibility so that inlining
+            --  and the analysis of instance bodies can proceed cleanly.
+
+            elsif Current_Sem_Unit = Main_Unit
+              and then Serious_Errors_Detected = 0
+              and then not Implicit_With (Item)
+            then
+               Set_Is_Immediately_Visible
+                 (Defining_Entity  (Unit (Library_Unit (Item))), False);
+            end if;
          end if;
 
          Next (Item);