[Ada] Fix crash on extension of private type with -gnatRj

[Pierre-Marie de Rodat]

This fixes a crash (or an assertion failure) during the processing done
for -gnatRj on the declaration of an extension of a private type.
Generally speaking, extension declarations are delicate in this context
because the front-end does not duplicate the structure of the parent
type, so the processing required to output the structural layout needs
to go up to the declaration of the parent type, which may or may not be
available or usable.

The change also makes the processing more robust by falling back to the
flat layout if the declaration of the parent type cannot be processed.

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

2019-07-08  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * repinfo.adb (List_Record_Info): Declare Incomplete_Layout and
        Not_In_Extended_Main local exceptions.
        (List_Structural_Record_Layout): For an extension, raise the
        former if the parent subtype has not been built and the latter
        if it is not declared in the main source unit.  Fall back to the
        flat layout if either exception has been raised.
--- gcc/ada/repinfo.adb
+++ gcc/ada/repinfo.adb
@@ -1125,6 +1125,12 @@ package body Repinfo is
          Indent    : Natural := 0);
       --  Internal recursive procedure to display the structural layout
 
+      Incomplete_Layout : exception;
+      --  Exception raised if the layout is incomplete in -gnatc mode
+
+      Not_In_Extended_Main : exception;
+      --  Exception raised when an ancestor is not declared in the main unit
+
       Max_Name_Length : Natural := 0;
       Max_Spos_Length : Natural := 0;
 
@@ -1564,14 +1570,29 @@ package body Repinfo is
 
                Disc        : Entity_Id;
                Listed_Disc : Entity_Id;
+               Parent_Type : Entity_Id;
 
             begin
                --  If this is an extension, first list the layout of the parent
                --  and then proceed to the extension part, if any.
 
                if Is_Extension then
-                  List_Structural_Record_Layout
-                    (Base_Type (Parent_Subtype (Ent)), Outer_Ent);
+                  Parent_Type := Parent_Subtype (Ent);
+                  if No (Parent_Type) then
+                     raise Incomplete_Layout;
+                  end if;
+
+                  if Is_Private_Type (Parent_Type) then
+                     Parent_Type := Full_View (Parent_Type);
+                     pragma Assert (Present (Parent_Type));
+                  end if;
+
+                  Parent_Type := Base_Type (Parent_Type);
+                  if not In_Extended_Main_Source_Unit (Parent_Type) then
+                     raise Not_In_Extended_Main;
+                  end if;
+
+                  List_Structural_Record_Layout (Parent_Type, Outer_Ent);
                   First := False;
 
                   if Present (Record_Extension_Part (Definition)) then
@@ -1733,8 +1754,23 @@ package body Repinfo is
          Write_Line (",");
          Write_Str ("  ""record"": [");
 
+         --  ??? We can output structural layout only for base types fully
+         --  declared in the extended main source unit for the time being,
+         --  because otherwise declarations might not be processed at all.
+
          if Is_Base_Type (Ent) then
-            List_Structural_Record_Layout (Ent, Ent);
+            begin
+               List_Structural_Record_Layout (Ent, Ent);
+
+            exception
+               when Incomplete_Layout
+                  | Not_In_Extended_Main
+               =>
+                  List_Record_Layout (Ent);
+
+               when others =>
+                  raise Program_Error;
+            end;
          else
             List_Record_Layout (Ent);
          end if;