!ASIS Issue #088
!topic Nil_Element as Corresponding_Expression_Type
!reference  ASIS 95-17.1
!from  Sergey Rybin 98-01-14
!keywords Nil_Element Corresponding_Expression_Type expression type
!discussion

There are additional situations where Nil_Element should be returned
by Corresponding_Expression_Type.  Nil_Element should be returned for
an enumeration_aggregate which is part of
enumeration_representation_clause.  There may be other kinds of
expressions where Nil_Element should be returned.

A general rule should be provided in the ASIS specification for the
return of Nil_Element in the case when an Expression actually does not
represent an Ada expression having an Ada type.  I would only point
out some important cases when Nil_Element should be returned, without
caring too much about the completeness of this list, and without
pretending to provide the complete list.

!resolution  Accept
!date 98-03-26
!Notes

Replace the following from Clause 17.1:

-- Returns a Nil_Element for the following classes of expressions that do not
-- have a Corresponding_Expression_Type:
--
--  - Naming expressions that name packages, subprograms, tasks, etc.  These
--    expressions do have a Corresponding_Name_Definition and a 
--    Corresponding_Name_Declaration.
--
--  - When An_Identifier Element representing an attribute designator is passed
--    as the actual to this query.
-- 
--  - The Actual_Parameter Expression from A_Pragma_Argument_Association for a
--    Pragma may or may not have a Corresponding_Expression_Type.  If the 
--    returned declaration is non-nil then the Expression is either:
--
--       a) an ordinary expression, or 
--       b) a naming expression that names a constant or variable object.
--
-- Returns a Nil_Element, if statically determinable type of Expression is a
-- class-wide type.

with:

-- Returns a Nil_Element if the argument Expression does not represent an Ada
-- expression having an Ada type, including the following classes:
--
--  - Naming expressions that name packages, subprograms, tasks, etc.  These
--    expressions do have a Corresponding_Name_Definition and a
--    Corresponding_Name_Declaration.
--
--  - When An_Identifier Element representing an attribute designator is passed
--    as the actual to this query.
--
--  - The Actual_Parameter Expression from A_Pragma_Argument_Association for a
--    Pragma may or may not have a Corresponding_Expression_Type.
--
--  - An_Attribute_Reference Element also may or may not have a
--    Corresponding_Expression_Type;
--
--  - An enumeration_aggregate which is a part of
--    enumeration_representation_clause.
--
-- Returns a Nil_Element, if statically determinable type of Expression is a
-- class-wide type.

Note: See Comment 103 which adds text to the first bullet above.