work around Converter virtual subclass support

add test for warning or error when a converter
supports multiple tags but does not implement
select_tag

asdf/_tests/test_extension.py

@@ -913,11 +913,14 @@ def from_yaml_tree(self, node, tag, ctx):
                 roundtrip_object(obj)
 
 
-def test_warning_for_default_select_tag():
+@pytest.mark.parametrize("is_subclass", [True, False])
+def test_warning_or_error_for_default_select_tag(is_subclass):
     class Foo:
         pass
 
-    class FooConverter(Converter):
+    ParentClass = Converter if is_subclass else object
+
+    class FooConverter(ParentClass):
         tags = ["asdf://somewhere.org/tags/foo-*"]
         types = [Foo]
 
@@ -932,6 +935,8 @@ def from_yaml_tree(self, node, tag, ctx):
         "asdf://somewhere.org/tags/foo-2.0.0",
     ]
     extension = FullExtension(converters=[FooConverter()], tags=tags)
+    ctx_type = pytest.warns if is_subclass else pytest.raises
+    exception_class = AsdfWarning if is_subclass else RuntimeError
     with config_context() as config:
-        with pytest.warns(AsdfWarning, match="Converter handles multiple tags"):
+        with ctx_type(exception_class, match="Converter handles multiple tags"):
             config.add_extension(extension)

asdf/extension/_converter.py

@@ -170,7 +170,9 @@ def __init__(self, delegate, extension):
                 raise TypeError(msg)
 
         if len(relevant_tags) > 1 and not hasattr(delegate, "select_tag"):
-            if isinstance(delegate, Converter):
+            # we cannot use isinstance here because Converter supports
+            # virtual subclasses
+            if Converter in delegate.__class__.__bases__:
                 # prior to asdf 3.0 Converter provided a default select_tag
                 # to provide backwards compatibility allow Converter subclasses
                 # to be registered with >1 tag but produce a warning