[Ada] Slice length computation bug in Generic_Bounded_Length generics

In some cases involving null slices, the Slice subprograms (both the
  function and the procedure) in each of the three Generic_Bounded_Length
  generic packages (for String, Wide_String, and Wide_Wide_String)
  could raise Constraint_Error in cases where this is incorrect.

gcc/ada/

	* libgnat/a-strsup.adb, libgnat/a-stwisu.adb, libgnat/a-stzsup.adb
	(Super_Slice function and procedure): fix slice length computation.

3 files changed, 16 insertions, 21 deletions

diff --git a/gcc/ada/libgnat/a-strsup.adb b/gcc/ada/libgnat/a-strsup.adb
index e301564..831a18e 100644
--- a/gcc/ada/libgnat/a-strsup.adb
+++ b/gcc/ada/libgnat/a-strsup.adb
@@ -1651,10 +1651,9 @@ package body Ada.Strings.Superbounded with SPARK_Mode is
             raise Index_Error;
          end if;
 
-         if High >= Low then
-            Result.Data (1 .. High - Low + 1) := Source.Data (Low .. High);
-            Result.Current_Length := High - Low + 1;
-         end if;
+         Result.Current_Length := (if Low > High then 0 else High - Low + 1);
+         Result.Data (1 .. Result.Current_Length) :=
+           Source.Data (Low .. High);
       end return;
    end Super_Slice;
 
@@ -1671,12 +1670,8 @@ package body Ada.Strings.Superbounded with SPARK_Mode is
          raise Index_Error;
       end if;
 
-      if High >= Low then
-         Target.Data (1 .. High - Low + 1) := Source.Data (Low .. High);
-         Target.Current_Length := High - Low + 1;
-      else
-         Target.Current_Length := 0;
-      end if;
+      Target.Current_Length := (if Low > High then 0 else High - Low + 1);
+      Target.Data (1 .. Target.Current_Length) := Source.Data (Low .. High);
    end Super_Slice;
 
    ----------------
diff --git a/gcc/ada/libgnat/a-stwisu.adb b/gcc/ada/libgnat/a-stwisu.adb
index a615ff3..d325676 100644
--- a/gcc/ada/libgnat/a-stwisu.adb
+++ b/gcc/ada/libgnat/a-stwisu.adb
@@ -1497,7 +1497,7 @@ package body Ada.Strings.Wide_Superbounded is
             raise Index_Error;
          end if;
 
-         Result.Current_Length := High - Low + 1;
+         Result.Current_Length := (if Low > High then 0 else High - Low + 1);
          Result.Data (1 .. Result.Current_Length) := Source.Data (Low .. High);
       end return;
    end Super_Slice;
@@ -1513,10 +1513,10 @@ package body Ada.Strings.Wide_Superbounded is
         or else High > Source.Current_Length
       then
          raise Index_Error;
-      else
-         Target.Current_Length := High - Low + 1;
-         Target.Data (1 .. Target.Current_Length) := Source.Data (Low .. High);
       end if;
+
+      Target.Current_Length := (if Low > High then 0 else High - Low + 1);
+      Target.Data (1 .. Target.Current_Length) := Source.Data (Low .. High);
    end Super_Slice;
 
    ----------------
diff --git a/gcc/ada/libgnat/a-stzsup.adb b/gcc/ada/libgnat/a-stzsup.adb
index d973993..6153bbe 100644
--- a/gcc/ada/libgnat/a-stzsup.adb
+++ b/gcc/ada/libgnat/a-stzsup.adb
@@ -1498,11 +1498,11 @@ package body Ada.Strings.Wide_Wide_Superbounded is
            or else High > Source.Current_Length
          then
             raise Index_Error;
-         else
-            Result.Current_Length := High - Low + 1;
-            Result.Data (1 .. Result.Current_Length) :=
-              Source.Data (Low .. High);
          end if;
+
+         Result.Current_Length := (if Low > High then 0 else High - Low + 1);
+         Result.Data (1 .. Result.Current_Length) :=
+           Source.Data (Low .. High);
       end return;
    end Super_Slice;
 
@@ -1517,10 +1517,10 @@ package body Ada.Strings.Wide_Wide_Superbounded is
         or else High > Source.Current_Length
       then
          raise Index_Error;
-      else
-         Target.Current_Length := High - Low + 1;
-         Target.Data (1 .. Target.Current_Length) := Source.Data (Low .. High);
       end if;
+
+      Target.Current_Length := (if Low > High then 0 else High - Low + 1);
+      Target.Data (1 .. Target.Current_Length) := Source.Data (Low .. High);
    end Super_Slice;
 
    ----------------