autodoc: Test the signature of typing.Generic subclasses.

This test is currently failing because typing.Generic.__new__ clobbers the
real signature.

tests/test_ext_autodoc.py

@@ -290,6 +290,36 @@ def foo3(self, d='\n'):
         '(b, c=42, *d, **e)'
 
 
+@pytest.mark.skipif(sys.version_info < (3, 5), reason='typing is available since python3.5.')
+@pytest.mark.xfail
+def test_autodoc_process_signature_typing_generic(app):
+    import typing
+
+    # Test that typing.Generic's __new__ method does not mask our class'
+    # __init__ signature.
+    T = typing.TypeVar('T')
+
+    class A(typing.Generic[T]):
+        def __init__(self, a, b=None):
+            pass
+
+    directive = make_directive_bridge(app.env)
+
+    inst = app.registry.documenters['class'](directive, 'A')
+    inst.fullname = 'A'
+    inst.doc_as_attr = False  # for class objtype
+    inst.parent = object  # dummy
+    inst.object = A
+    inst.objpath = ['A']
+    inst.args = None
+    inst.retann = None
+
+    sig = inst.format_signature()
+    print(sig)
+
+    assert sig == '(a, b=None)'
+
+
 def test_autodoc_process_signature_typehints(app):
     captured = []