Make sure that the builtin docstring works again for infer calls

conftest.py

@@ -120,6 +120,11 @@ def goto_or_help(request, Script):
     return lambda code, *args, **kwargs: getattr(Script(code), request.param)(*args, **kwargs)
 
 
+@pytest.fixture(scope='session', params=['goto', 'help', 'infer'])
+def goto_or_help_or_infer(request, Script):
+    return lambda code, *args, **kwargs: getattr(Script(code), request.param)(*args, **kwargs)
+
+
 @pytest.fixture(scope='session')
 def has_typing(environment):
     if environment.version_info >= (3, 5, 0):

jedi/inference/names.py

@@ -228,7 +228,12 @@ class ValueNameMixin(object):
         return ValueSet([self._value])
 
     def py__doc__(self):
-        return self._value.py__doc__()
+        doc = self._value.py__doc__()
+        if not doc and self._value.is_stub():
+            from jedi.inference.gradual.conversion import convert_names
+            names = convert_names([self], prefer_stub_to_compiled=False)
+            return _merge_name_docs(names)
+        return doc
 
     def _get_qualified_names(self):
         return self._value.get_qualified_names()

test/test_api/test_documentation.py

@@ -89,3 +89,11 @@ def test_version_info(Script):
 
     c, = s.completions()
     assert c.docstring() == 'sys.version_info\n\nVersion information as a named tuple.'
+
+
+def test_builtin_docstring(Script, goto_or_help_or_infer):
+    d, = goto_or_help_or_infer('open')
+
+    doc = d.docstring()
+    assert doc.startswith('open(file: Union[')
+    assert 'Open file' in doc or 'Open a file' in doc