[Ada] fix 'Read/'Write for unchecked_union

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

For an unchecked_union type, direct use of the default Read and Write
stream procedures raises program_error. However, Input and Output are
legal and meaningful when the type has default discriminants. When
streaming a value of such a type, we must use the default values directly
because we cannot write a reference to the (missing) discriminant field.

gnat.dg/unc.adb must compile quietly.

2007-04-06  Ed Schonberg  <schonberg@adacore.com>

	* exp_strm.adb
	(Build_Mutable_Record_Write_Procedure): For an Unchecked_Union type, use
	 discriminant defaults.
	(Build_Record_Or_Elementary_Output_Procedure): Ditto.
	(Make_Component_List_Attributes): Ditto.

Attachment: difs
Description: Text document