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[Ada] Size should be zero for null range discrete subtype

Message ID 20150522125041.GA25586@adacore.com
State New
Headers show

Commit Message

Arnaud Charlet May 22, 2015, 12:50 p.m. UTC 
The 'Size of a discrete subtype with a null range should be zero. The
following test:

     1. with Ada.Text_IO; use Ada.Text_IO;
     2. procedure Static_Null_Range_Size is
     3.    subtype Static_Null_Range is
     4.      Integer range 5 .. 0;
     5.    Dummy : Static_Null_Range;
     6. begin
     7.    Put_Line ("Static_Null_Range:");
     8.    Put_Line ("    Type'Size is " &
     9.                Natural'Image
    10.                (Static_Null_Range'Size));
    11.    Put_Line ("  Object'Size is " &
    12.                Natural'Image (Dummy'Size));
    13.    if Static_Null_Range'Size /= 0 then
    14.       raise Program_Error;
    15.    end if;
    16. end Static_Null_Range_Size;

should generate the output:

Static_Null_Range:
    Type'Size is  0
  Object'Size is  32

and not raise an exception.

Tested on x86_64-pc-linux-gnu, committed on trunk

2015-05-22  Robert Dewar  <dewar@adacore.com>

	* sem_ch13.adb (Minimum_Size): Size is zero for null range
	discrete subtype.
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Patch

Index: sem_ch13.adb
===================================================================
--- sem_ch13.adb	(revision 223555)
+++ sem_ch13.adb	(working copy)
@@ -11718,11 +11718,20 @@ 
          Lo := Uint_0;
       end if;
 
+      --  Null range case, size is always zero. We only do this in the discrete
+      --  type case, since that's the odd case that came up. Probably we should
+      --  also do this in the fixed-point case, but doing so causes peculiar
+      --  gigi failures, and it is not worth worrying about this incredibly
+      --  marginal case (explicit null-range fixed-point type declarations)???
+
+      if Lo > Hi and then Is_Discrete_Type (T) then
+         S := 0;
+
       --  Signed case. Note that we consider types like range 1 .. -1 to be
       --  signed for the purpose of computing the size, since the bounds have
       --  to be accommodated in the base type.
 
-      if Lo < 0 or else Hi < 0 then
+      elsif Lo < 0 or else Hi < 0 then
          S := 1;
          B := Uint_1;