[Ada] Fix PR ada/98230

[Eric Botcazou]

It's a rather curious malfunction of the 'Mod attribute applied to the 
variable of a loop whose upper bound is dynamic.

Tested on x86-64/Linux, applied on all active branches.


2020-12-10  Ed Schonberg  <schonb...@adacore.com>

        PR ada/98230
        * exp_attr.adb (Expand_N_Attribute_Reference, case Mod): Use base
        type of argument to obtain static bound and required size.


2020-12-10  Eric Botcazou  <ebotca...@adacore.com>

        * gnat.dg/modular6.adb: New test.

-- 
Eric Botcazou
diff --git a/gcc/ada/exp_attr.adb b/gcc/ada/exp_attr.adb
index 251fa1449c4..b21592c78b2 100644
--- a/gcc/ada/exp_attr.adb
+++ b/gcc/ada/exp_attr.adb
@@ -4702,13 +4702,15 @@ package body Exp_Attr is
 
       when Attribute_Mod => Mod_Case : declare
          Arg  : constant Node_Id := Relocate_Node (First (Exprs));
-         Hi   : constant Node_Id := Type_High_Bound (Etype (Arg));
+         Hi   : constant Node_Id := Type_High_Bound (Base_Type (Etype (Arg)));
          Modv : constant Uint    := Modulus (Btyp);
 
       begin
 
          --  This is not so simple. The issue is what type to use for the
-         --  computation of the modular value.
+         --  computation of the modular value. In addition we need to use
+         --  the base type as above to retrieve a static bound for the
+         --  comparisons that follow.
 
          --  The easy case is when the modulus value is within the bounds
          --  of the signed integer type of the argument. In this case we can

-- { dg-do compile }

with Ada.Text_IO; use Ada.Text_IO;

procedure Modular6 is
   Max : Integer := 0;
   
   type Modulus is mod 3;
begin
   Max := 30;
   
   for N in 1 .. Max loop
      Put_Line("N: " & Integer'Image(N) & " Modulus:    " & Integer'Image(Modulus'Modulus) & " Mod:" & Modulus'Image(Modulus'Mod(N)));
   end loop;
end;