[Ada] Warn on not (not ..) if -gnatwr active

[Ada] Warn on not (not ..) if -gnatwr active

[Arnaud Charlet]

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Tested on i686-linux, committed on trunk

The construct not (not ...) is now included as one of the redundant
constructs that generate a warning if -gnatwr is used:

     1. procedure notnot (x, y : integer) is
     2. begin
     3.    if not (not (x = y)) then
              |
        >>> warning: redundant double negation

     4.       return;
     5.    end if;
     6. end;

See gnat.dg/notnot.adb

2008-05-20  Robert Dewar  <dewar@adacore.com>

	* sem_res.adb (Resolve_Op_Not): Warn on double negation


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Index: sem_res.adb
===================================================================
--- sem_res.adb	(revision 134945)
+++ sem_res.adb	(working copy)
@@ -6975,6 +6975,19 @@ package body Sem_Res is
             Error_Msg_N ("?not expression should be parenthesized here!", N);
          end if;
 
+         --  Warn on double negation if checking redundant constructs
+
+         if Warn_On_Redundant_Constructs
+           and then Comes_From_Source (N)
+           and then Comes_From_Source (Right_Opnd (N))
+           and then Root_Type (Typ) = Standard_Boolean
+           and then Nkind (Right_Opnd (N)) = N_Op_Not
+         then
+            Error_Msg_N ("redundant double negation?", N);
+         end if;
+
+         --  Complete resolution and evaluation of NOT
+
          Resolve (Right_Opnd (N), B_Typ);
          Check_Unset_Reference (Right_Opnd (N));
          Set_Etype (N, B_Typ);