[Ada] Implement fixed-lower-bound consistency checks for qualified_expressions

[Pierre-Marie de Rodat]

This change implements a missing check for qualified_expressions where
the qualifying subtype is an unconstrained array subtype that specifies
fixed lower bounds for one or more of its index ranges.

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

gcc/ada/

        * checks.adb (Selected_Range_Checks): In the case of a
        qualified_expression where the qualifying subtype is an
        unconstrained array subtype with fixed lower bounds for some of
        its indexes, generate tests to check that those bounds are equal
        to the corresponding lower bounds of the qualified array object.
diff --git a/gcc/ada/checks.adb b/gcc/ada/checks.adb
--- a/gcc/ada/checks.adb
+++ b/gcc/ada/checks.adb
@@ -11106,6 +11106,56 @@ package body Checks is
                end;
             end if;
 
+         --  If the context is a qualified_expression where the subtype is
+         --  an unconstrained array subtype with fixed-lower-bound indexes,
+         --  then consistency checks must be done between the lower bounds
+         --  of any such indexes and the corresponding lower bounds of the
+         --  qualified array object.
+
+         elsif Is_Fixed_Lower_Bound_Array_Subtype (T_Typ)
+           and then Nkind (Parent (Expr)) = N_Qualified_Expression
+           and then not Do_Access
+         then
+            declare
+               Ndims : constant Pos := Number_Dimensions (T_Typ);
+
+               Qual_Index : Node_Id;
+               Expr_Index : Node_Id;
+
+            begin
+               Expr_Actual := Get_Referenced_Object (Expr);
+               Exptyp      := Get_Actual_Subtype (Expr_Actual);
+
+               Qual_Index := First_Index (T_Typ);
+               Expr_Index := First_Index (Exptyp);
+
+               for Indx in 1 .. Ndims loop
+                  if Nkind (Expr_Index) /= N_Raise_Constraint_Error then
+
+                     --  If this index of the qualifying array subtype has
+                     --  a fixed lower bound, then apply a check that the
+                     --  corresponding lower bound of the array expression
+                     --  is equal to it.
+
+                     if Is_Fixed_Lower_Bound_Index_Subtype (Etype (Qual_Index))
+                     then
+                        Evolve_Or_Else
+                          (Cond,
+                           Make_Op_Ne (Loc,
+                             Left_Opnd   =>
+                               Get_E_First_Or_Last
+                                 (Loc, Exptyp, Indx, Name_First),
+                             Right_Opnd  =>
+                               New_Copy_Tree
+                                 (Type_Low_Bound (Etype (Qual_Index)))));
+                     end if;
+
+                     Next (Qual_Index);
+                     Next (Expr_Index);
+                  end if;
+               end loop;
+            end;
+
          else
             --  For a conversion to an unconstrained array type, generate an
             --  Action to check that the bounds of the source value are within