Type of the target of an assignment shall not be abstract

[Gautier write-only <gautier_niouzes@hotmail.com>]

Hello,

A question for the RM hermeneutists.
The instruction marked below with ">>>" goes well through one Ada 95
compiler (GNAT with -gnat95) but is not accepted by another one
(ObjectAda 7.2.2).
Which compiler is right ?

  procedure Init(
    xl        : in out Excel_Out_Stream'Class;
    format    :        Excel_type:= Default_Excel_type
  )
  is
    dummy_xl_with_defaults: Excel_Out_File;
  begin
    -- Check if we are trying to re-use a half-finished object
(ouch!):
    if xl.is_created and not xl.is_closed then
      raise Excel_Stream_Not_Closed;
    end if;
    dummy_xl_with_defaults.format:= format;
>>> Excel_Out_Stream(xl):= Excel_Out_Stream(dummy_xl_with_defaults);
  end Init;

>>> excel_out.adb: Error: line 595 col 5 LRM:3.9.3(8), The type of the target of an assignment shall not be abstract

For those asking themselves what the *@#$! that code is doing, since
the xl variable is already initialized, the purpose of the := is to re-
initialize an object (xl) with the default values of the root type.
Since the root type is abstract, I need a concrete derived object
(dummy_xl_with_defaults) to get these default values. Subsidiary
question: is there another way of doing that in one ':=' ?

The whole package is visible @ http://excel-writer.sourceforge.net/ew_html/excel_out__adb.htm#553_13
______________________________________________________________
Gautier's Ada programming -- http://gautiersblog.blogspot.com/