[Ada] Spurious discriminant check on bounded synchronized queue

[Pierre-Marie de Rodat]

An improper run-time error is raised on a bounded synchronized queue
when the container element is a discriminated type that is itself a
bounded container.

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

gcc/ada/

        * libgnat/a-cbsyqu.ads (Implementation): Provide a box
        initialization for the element array used internally to
        represent the queue, so that its components are properly
        initialized if the given element type has default
        initialization. Suppress warnings on the rest of the package in
        case the element type has no default or discriminant, because it
        is bound to be confusing to the user.
diff --git a/gcc/ada/libgnat/a-cbsyqu.ads b/gcc/ada/libgnat/a-cbsyqu.ads
--- a/gcc/ada/libgnat/a-cbsyqu.ads
+++ b/gcc/ada/libgnat/a-cbsyqu.ads
@@ -71,6 +71,14 @@ is
 
       --  Need proper heap data structure here ???
 
+      --  We suppress warnings here, which might otherwise be triggered
+      --  by the box initialization of the Elements array below. This
+      --  initialization is needed to preserve constraints, such as
+      --  discriminant values, that the actual for Element_Type might
+      --  carry.
+
+      pragma Warnings (Off);
+
       type Element_Array is
         array (Count_Type range <>) of Queue_Interfaces.Element_Type;
 
@@ -78,7 +86,7 @@ is
          First, Last : Count_Type := 0;
          Length      : Count_Type := 0;
          Max_Length  : Count_Type := 0;
-         Elements    : Element_Array (1 .. Capacity);
+         Elements    : Element_Array (1 .. Capacity) := (others => <>);
       end record;
 
    end Implementation;