gtype: Add g_type_interface_instantiable_prerequisite()

There is (at most) a single GType that is instantiable and a
prerequisite for an interface. This function returns that type.

This type is necessary in particular when dealing with GValues because a
GValue contains an instance of a type.

docs/reference/gobject/gobject-sections.txt

@@ -65,6 +65,7 @@ g_type_default_interface_unref
 g_type_children
 g_type_interfaces
 g_type_interface_prerequisites
+g_type_interface_instantiable_prerequisite
 g_type_set_qdata
 g_type_get_qdata
 g_type_query

gobject/gtype.c

@@ -1689,6 +1689,51 @@ g_type_interface_prerequisites (GType  interface_type,
     }
 }
 
+/**
+ * g_type_interface_instantiable_prerequisite:
+ * @interface_type: an interface type
+ *
+ * Returns the instantiable prerequisite of an interface type.
+ *
+ * If the interface type has no instantiable prerequisite, 0 is returned.
+ *
+ * Since: 2.64
+ *
+ * Returns: the instantiable prerequisite type or 0 if none
+ **/
+GType
+g_type_interface_instantiable_prerequisite (GType interface_type)
+{
+  TypeNode *inode = NULL;
+  TypeNode *iface;
+  guint i;
+
+  g_return_val_if_fail (G_TYPE_IS_INTERFACE (interface_type), G_TYPE_INVALID);
+
+  iface = lookup_type_node_I (interface_type);
+  if (iface == NULL)
+    return 0;
+
+  G_READ_LOCK (&type_rw_lock);
+
+  for (i = 0; i < IFACE_NODE_N_PREREQUISITES (iface); i++)
+    {
+      GType prerequisite = IFACE_NODE_PREREQUISITES (iface)[i];
+      TypeNode *node = lookup_type_node_I (prerequisite);
+      if (node->is_instantiatable)
+        {
+          if (!inode || type_node_is_a_L (node, inode))
+            inode = node;
+        }
+    }
+
+  G_READ_UNLOCK (&type_rw_lock);
+
+  if (inode)
+    return NODE_TYPE (inode);
+  else
+    return 0;
+}
 
 static IFaceHolder*
 type_iface_peek_holder_L (TypeNode *iface,

gobject/gtype.h

@@ -1304,6 +1304,9 @@ void  g_type_interface_add_prerequisite (GType			     interface_type,
 GLIB_AVAILABLE_IN_ALL
 GType*g_type_interface_prerequisites    (GType                       interface_type,
 					 guint                      *n_prerequisites);
+GLIB_AVAILABLE_IN_2_64
+GType g_type_interface_instantiable_prerequisite
+                                        (GType                       interface_type);
 GLIB_DEPRECATED_IN_2_58
 void     g_type_class_add_private       (gpointer                    g_class,
                                          gsize                       private_size);

gobject/tests/type.c

@@ -52,6 +52,7 @@ test_interface_prerequisite (void)
   g_assert_cmpint (n_prereqs, ==, 2);
   g_assert (prereqs[0] == bar_get_type ());
   g_assert (prereqs[1] == G_TYPE_OBJECT);
+  g_assert (g_type_interface_instantiable_prerequisite (foo_get_type ()) == G_TYPE_OBJECT);
 
   iface = g_type_default_interface_ref (foo_get_type ());
   parent = g_type_interface_peek_parent (iface);