ada: Missing discriminant check on assignment of Bounded_Vector aggregate

When a container aggregate for a Bounded_Vector type involves an iterated
association that is assigned to a vector object whose capacity (as defined
by the Capacity discriminant) is less than the number of elements of the
aggregate, Constraint_Error should be raised due to failing a discriminant
check on the assignment. But the compiler fails to do proper expansion,
plus omits the check, and instead creates a temporary whose capacity is
bounded by that of the target vector of the assignment. It attempts to
assign all elements of the aggregate to the temporary, resulting in
a failure on a call to the Replace_Element operation that assigns past
the length of the temporary vector (which can result in a Storage_Error
due to a segment violation).

This is fixed by ensuring that the temporary object is declared with
an unconstrained base subtype rather than the assignment target's
constrained subtype.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Expand_Container_Aggregate): Use the Base_Type of the
	subtype provided by the context as the subtype of the temporary object
	initialized by the aggregate.

gcc/ada/exp_aggr.adb

diff --git a/gcc/ada/exp_aggr.adb b/gcc/ada/exp_aggr.adb
index 8f1869c..f4674f5 100644
--- a/gcc/ada/exp_aggr.adb
+++ b/gcc/ada/exp_aggr.adb
@@ -7496,10 +7496,19 @@
          Set_Assignment_OK (Lhs);
 
          Aggr_Code := Build_Container_Aggr_Code (N, Typ, Lhs, Init);
+
+         --  Use the unconstrained base subtype of the subtype provided by
+         --  the context for declaring the temporary object (which may come
+         --  from a constrained assignment target), to ensure that the
+         --  aggregate can be successfully expanded and assigned to the
+         --  temporary without exceeding its capacity. (Later assignment
+         --  of the temporary to a target object may result in failing
+         --  a discriminant check.)
+
          Prepend_To (Aggr_Code,
            Make_Object_Declaration (Loc,
              Defining_Identifier => Obj_Id,
-             Object_Definition   => New_Occurrence_Of (Typ, Loc),
+             Object_Definition   => New_Occurrence_Of (Base_Type (Typ), Loc),
              Expression          => Init));
 
          Insert_Actions (N, Aggr_Code);