[Ada] Vectors: spurious error in -gnatwE mode

This patch fixes a bug in which if Ada.Containers.Vectors is instantiated with
an Index_Type such that Index_Type'Base'Last is less than Count_Type'Last, and
the -gnatwE switch is used, the compiler gives spurious error messages.

The following test should compile quietly with -gnatwE:

gnatmake short_vectors.ads -gnatwa -gnatwE -gnatf

with Ada.Containers.Vectors;
package Short_Vectors is

   type Index_Type is range 1 .. 256;

   package Map_Pkg is new Ada.Containers.Vectors
     (Index_Type => Index_Type,
      Element_Type => Integer);

end Short_Vectors;

2018-05-23  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-convec.adb: (Insert, Insert_Space): Suppress warnings. The
	code in question is not reachable in the case where Count_Type'Last is
	out of range.

From-SVN: r260590

gcc/ada/ChangeLog

+2018-05-23  Bob Duff  <duff@adacore.com>
+
+	* libgnat/a-convec.adb: (Insert, Insert_Space): Suppress warnings. The
+	code in question is not reachable in the case where Count_Type'Last is
+	out of range.
+
 2018-05-23  Yannick Moy  <moy@adacore.com>
 
 	* doc/gnat_rm/implementation_defined_pragmas.rst: Clarify meaning of

gcc/ada/libgnat/a-convec.adb

@@ -999,9 +999,12 @@ package body Ada.Containers.Vectors is
 
             --  We know that No_Index (the same as Index_Type'First - 1) is
             --  less than 0, so it is safe to compute the following sum without
-            --  fear of overflow.
+            --  fear of overflow. We need to suppress warnings, because
+            --  otherwise we get an error in -gnatwE mode.
 
+            pragma Warnings (Off);
             Index := No_Index + Index_Type'Base (Count_Type'Last);
+            pragma Warnings (On);
 
             if Index <= Index_Type'Last then
 
@@ -1657,9 +1660,12 @@ package body Ada.Containers.Vectors is
 
             --  We know that No_Index (the same as Index_Type'First - 1) is
             --  less than 0, so it is safe to compute the following sum without
-            --  fear of overflow.
+            --  fear of overflow. We need to suppress warnings, because
+            --  otherwise we get an error in -gnatwE mode.
 
+            pragma Warnings (Off);
             Index := No_Index + Index_Type'Base (Count_Type'Last);
+            pragma Warnings (On);
 
             if Index <= Index_Type'Last then