varobj/Ada: Missing children for interface-wide tagged types

Consider the following code:

   type Element is abstract tagged null record;
   type GADataType is interface;
   type Data_Type is new Element and GADataType with record
      I : Integer := 42;
   end record;
   Result1 : Data_Type;
   GGG1    : GADataType'Class := GADataType'Class (Result1);

When trying to create a varobj for variable ggg1, GDB currently
returns an object which has no child:

    -var-create ggg1 * ggg1
    ^done,name="ggg1",numchild="0",[...]

This is incorrect, it should return an object which has one child
(field "i"). This is because tagged-type objects are dynamic, and
we need to apply a small transformation in order to get their actual
type. This is already done on the GDB/CLI side in ada-valprint,
and it needs to be done on the ada-varobj side as well.

gdb/ChangeLog:

        * ada-varobj.c (ada_varobj_adjust_for_child_access): Convert
        tagged type objects to their actual type.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_interface: New testcase.

1 files changed, 9 insertions, 0 deletions

diff --git a/gdb/ada-varobj.c b/gdb/ada-varobj.c
index aab7335..3da6018 100644
--- a/gdb/ada-varobj.c
+++ b/gdb/ada-varobj.c
@@ -219,6 +219,15 @@ ada_varobj_adjust_for_child_access (struct value **value,
       && !ada_is_array_descriptor_type (TYPE_TARGET_TYPE (*type))
       && !ada_is_constrained_packed_array_type (TYPE_TARGET_TYPE (*type)))
     ada_varobj_ind (*value, *type, value, type);
+
+  /* If this is a tagged type, we need to transform it a bit in order
+     to be able to fetch its full view.  As always with tagged types,
+     we can only do that if we have a value.  */
+  if (*value != NULL && ada_is_tagged_type (*type, 1))
+    {
+      *value = ada_tag_value_at_base_address (*value);
+      *type = value_type (*value);
+    }
 }
 
 /* Assuming that the (PARENT_VALUE, PARENT_TYPE) pair is an array