P'Base definition vs. usage in LRM

Capability/request summary

This is a Bug Report + Feature Request rolled into one.

Attribute definition for P'base says:

P'BASE Kind: Type.
Prefix: Any prefix P that is appropriate for an object with type or subtype T,
or an alias thereof, or that denotes any type or subtype T.
Result: The base type of T.
Restrictions: This attribute is allowed only as the prefix of the name of another attribute; for example, P'BASE'LEFT.

The restriction is not phrased correctly. If it is meant to be an error, it should state that it is an error.

Restrictions: It is an error if this attribute is as anything other than as the prefix to another attribute; for example, P'BASE'LEFT.

from 5.3.2.1 page 61 of 1076-2019 says This is in a note

NOTE—The rules concerning constrained type declarations mean that a type declaration with a constrained array definition such as:

type T is array (POSITIVE range MIN_BOUND to MAX_BOUND) of ELEMENT;

is equivalent to the sequence of declarations

subtype index_subtype is POSITIVE range MIN_BOUND to MAX_BOUND;
type array_type is array (index_subtype range <>) of ELEMENT'BASE;
subtype T is array_type (index_subtype)element_constraint;

where index_subtype and array_type are both anonymous and element_constraint is the constraint that applies to the subtype ELEMENT. Consequently, T is the name of a subtype and all objects declared with this type mark are arrays that have the same index range.

Similarly, a type declaration with an unbounded array definition whose element subtype indication denotes a partially or fully constrained subtype such as

type T is array (INTEGER range <>) of STRING(1 to 10);

is equivalent to the sequence of declarations

type array_type is array (INTEGER range <>) of STRING'BASE;
subtype T is array_type (open)(1 to 10);

While the note is not normative, it should not be using an attribute in an illegal way.

Proposed Changes

Add an array based version:

A'BASE Kind: Subtype.
Prefix: Any prefix A that is appropriate for an array object, or an alias
thereof, or that denotes an array subtype.
Result: The base type of A.

Or Alternately updated P'Base as

P'BASE Kind: Type.
Prefix: Any prefix P that is appropriate for an object with type or subtype T,
or an alias thereof, or that denotes any type or subtype T.
Result: The base type of T.
Restrictions: When the result type T is not an array type, this attribute is allowed only as the prefix of the name of another attribute; for example, P'BASE'LEFT.

Or based on Patrick's suggestion, update P'Base without restrictions:

P'BASE Kind: Type.
Prefix: Any prefix P that is appropriate for an object with type or subtype T,
or an alias thereof, or that denotes any type or subtype T.
Result: The base type of T.

Use Models | Code Examples

  function Reverse 
  ( A : type is array(type is range <>) of type is private ) return A'subtype is 
    variable Result : A'base(A'reverse_range) ;   
  begin
    for i in A'range loop 
      result(i) := A(i) ;
    end loop ; 
    return Result ; 
  end function Reverse ;

Notes

The above code currently works in one VHDL simulator.

Edited by JimLewis