[Ada] Error message on invalid conversion involving limited views

[Pierre-Marie de Rodat]

A type conversion may be illegal if the expression in the conversion has a
limited view of a type. This patch expands the error report to indicate the
presence of a limited view, and when the context is a package body it suggests
the addition of a regular with-clause to make the full view available.

Compiling client.adb must yield:

   client.adb:6:16: invalid conversion, not compatible with limited view
      of type "Map_Type" defined at maps.ads:2
   client.adb:6:16: add with_clause for "Maps" to current unit

----
package Maps is
  type Map_Type is null record;
end;
----
limited with Maps;
package Payloads is
  function Get_Map return access Maps.Map_Type;
end;
----
with Maps;
package Maps2 is
  type New_Map_Type is new Maps.Map_Type;
end;
----
with Maps2;
package Client is
  procedure Foo (Map : Maps2.New_Map_Type) is null;
  procedure Bar;
end;
----
with Payloads;
package body Client is
  procedure Bar is
  begin
     Foo (Maps2.New_Map_Type (Payloads.Get_Map.all));
  end;
end;

Tested on x86_64-pc-linux-gnu, committed on trunk

2018-05-21  Ed Schonberg  <schonb...@adacore.com>

gcc/ada/

        * sem_res.adb (Valid_Conversion): Improve error message on an illegal
        type conversion whose expression has a limited view of a type.
--- gcc/ada/sem_res.adb
+++ gcc/ada/sem_res.adb
@@ -13011,8 +13011,33 @@ package body Sem_Res is
       --  Here we have a real conversion error
 
       else
-         Conversion_Error_NE
-           ("invalid conversion, not compatible with }", N, Opnd_Type);
+
+         --  Check for missing regular with_clause when only a limited view
+         --  of target is available.
+
+         if From_Limited_With (Opnd_Type)
+           and then In_Package_Body
+         then
+            Conversion_Error_NE
+              ("invalid conversion, not compatible with limited view of }",
+                N, Opnd_Type);
+            Conversion_Error_NE ("\add with_clause for& to current unit!", N,
+              Scope (Opnd_Type));
+
+         elsif Is_Access_Type (Opnd_Type)
+           and then From_Limited_With (Designated_Type (Opnd_Type))
+           and then In_Package_Body
+         then
+            Conversion_Error_NE
+              ("invalid conversion, not compatible with }", N, Opnd_Type);
+            Conversion_Error_NE ("\add with_clause for& to current unit!", N,
+              Scope (Designated_Type (Opnd_Type)));
+
+         else
+            Conversion_Error_NE
+              ("invalid conversion, not compatible with }", N, Opnd_Type);
+         end if;
+
          return False;
       end if;
    end Valid_Conversion;