VHDL Issue Number:      0193
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                VPI Issue 17 -- Base types of constrained arrays
Related Issues:         0186
Relevant LRM Sections:  3.2.1
Key Words and Phrases:  Anonymous Type
Current Status:         Analyzed
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  LRM is clear now, as discussed in 3.2.1.
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/15
Author of Submission:   J. R. Armstrong
Author's Affiliation:   Virginia Polytechnic Institute and State University
Author's Post Address:  Bradley Department of Electrical Engineering
                        Virginia Polytechnic Institute and State University
                        Blacksburg, VA 24061
Author's Phone Number:  (703) 231-4723
Author's Fax Number:    
Author's Net Address:   JRA@VTVM1.CC.VT.EDU
-----------------------
Date Analyzed:          1991/12/05
Author of Analysis:     Clive Charlwood
Revision Number:        $Revision: 1.11 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
  Is the base type of a constrained array type an
  array type (unconstrained array definition) or an array sub-
  type (constrained array definition) ?

  Test File: ch14/s01/02020101.vhd

  The type of the prefix of the  attribute  'RIGHT,  'LOW,  or
  'LENGTH  must  be a constrained array subtype.  However, LRM
  doesn't explicitly specify whether the base type of  a  con-
  strained array type is constrained or not.


  --##E1002211145FF*********************************************
  --
  --+ File: [SUITE.CH14.S01]02020101.VHD
  --
  --+ Copyright: (c) 1986 by Intermetrics Inc. All rights reserved
  --
  --+ LRM_Version: IEEE Std. 1076-1987
  --
  --+ Subsets: None
  --
  --+ Description:
  --
  --+ Test_point -     (140100_020201)  The prefix of T'BASE may be any type or
  --                                     subtype T.
  --
  --+ Test_objective (1) - Test that the prefix T in T'BASE is any type or
  --  subtype.
  --
  --+ References: None
  --
  --+ Instructions: None
  --
  --+ Keywords: T'BASE, unconstrained array.
  --
  --+ Test_Results: Expect_Success. Self_Checking.
  --
  --+ Test_Type: Locally Static Semantic
  --
  --+ History: Created MM/MCC 8-15-89
  --           Modified KSC/VT 8-28-89
  --           Corrected 1-7-90
  --
  --+ Comments:  Original file name: S-14-1-0-0002A.VHD
  --             13-September-1988 14:10:20 by David Franke(Franke@mcc.com)
  --             Removed return in process
  --             Updated to 1076-1987 VHDL, checked w/Recognizer. (Steve)Gro
  --             out@mcc.com 20jun88
  --***********************************************************

  entity E  is
      port (PT:BOOLEAN) ;
  end E;

  architecture BB of E is
      signal S1 : BIT_VECTOR (0 to 15);
      signal S2 : INTEGER ;
   begin
    process
      subtype ST is INTEGER range -5 to 20;
      type E is (A,B,C,D);
      type P is range 1 to 24
          units
              U;
              X=3 U;
              Y=2 X;
          end units;
      type AR1 is array (-4 to -1,C downto A,BIT'BASE'LEFT to BIT'BASE'RIGHT)
      of ST;
      type AR2 is array ('X' to 'Z',TRUE downto FALSE) of INTEGER;
      variable V1 : INTEGER;
      variable V2 : BIT;
      variable V3 : E;
      variable V4 : P;
      variable V5 : REAL;
      variable V6 : BOOLEAN;
    begin
      V3 := E'BASE'LEFT;
      V2 := AR1'BASE'RIGHT(3);
      V5 := REAL'BASE'HIGH;
      V6 := AR2'BASE'LOW(2);
      V1 := E'BASE'POS(C);
      V1 := ST'BASE'VAL(1);
      V1 := INTEGER'BASE'PRED(1);
      V4 := P'BASE'SUCC(2 Y);
      V1 := AR1'BASE'LENGTH(2);
      S2 <= 8;

          assert FALSE
          report "TEST OK for the given Test Point"
          severity note;
          wait;

    end process;
  end BB;


Proposed Resolution
-------------------
TBD


VASG-ISAC Analysis & Rationale
------------------------------
LRM section 3.2.1 page 3-9 paragraph 5 specifies that:
    A constrained array definition defines both an array type and a subtype of
    this type:
    -   The array type is an implicitly declared anonymous type; this type is
        defined by an (implicit) unconstrained array definition, in which the
        element subtype indication is that of the constrained array definition,
        and in which the type mark of each index subtype definition denotes the
        subtype defined by the corresponding discrete range.
    -   The array subtype is the subtype obtained by imposition of the index
        constraint on the array type.
    If a constrained array definition is given for a type declaration, the
    simple name declared by this declaration denotes the array subtype.

From the above paragraph, it is clear that the names AR1, AR2 refer to
the constrained subtypes of the corresponding unconstrained anonymous base
type.

Note: This IR is similar to IR 0186. In both cases the type declaration 
      generates an unexpected anonymous type and a named subtype.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
No changes are needed to the LRM.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
When the language is modified for future revisions the designers should
consider whether it was possible to clean up this minor point. One
way to do this would be to define "special case" semantics for the 
'BASE attribute for a constrained array to do "what the user expects".
This does make the definition of 'BASE more complicated. This may not
be desirable.