Fix 'NoneType' object is not iterable on Generics

Fix the following exception during doc builds

```
Exception occurred:
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 119, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
TypeError: 'NoneType' object is not iterable
```

```
\# Sphinx version: 1.8.2
\# Python version: 3.7.1 (CPython)
\# Docutils version: 0.14
\# Jinja2 version: 2.10
\# Last messages:
\#   19 added, 0 changed, 0 removed
\#
\#   reading sources...
\#
\# Loaded extensions:
\#   sphinx.ext.mathjax (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/mathjax.py
\#   alabaster (0.7.12) from /path/to/.venv/lib/python3.7/site-packages/alabaster/__init__.py
\#   sphinx.ext.autodoc (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py
\#   sphinx.ext.intersphinx (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/intersphinx.py
\#   sphinx.ext.napoleon (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/napoleon/__init__.py
\#   sphinx_autodoc_typehints (unknown version) from /path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py
Traceback (most recent call last):
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 347, in build_update
    len(to_build))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 360, in build
    updated_docnames = set(self.read())
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 468, in read
    self._read_serial(docnames)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 490, in _read_serial
    self.read_doc(docname)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 534, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/io.py", line 318, in read_doc
    pub.publish()
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/core.py", line 217, in publish
    self.settings)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/readers/__init__.py", line 72, in read
    self.parse()
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/readers/__init__.py", line 78, in parse
    self.parser.parse(self.input, document)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/parsers.py", line 88, in parse
    self.statemachine.run(inputstring, document, inliner=self.inliner)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 171, in run
    input_source=document['source'])
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 239, in run
    context, state, transitions)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 460, in check_line
    return method(match, context, next_state)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2753, in underline
    self.section(title, source, style, lineno - 1, messages)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 327, in section
    self.new_subsection(title, lineno, messages)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 395, in new_subsection
    node=section_node, match_titles=True)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 282, in nested_parse
    node=node, match_titles=match_titles)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 196, in run
    results = StateMachineWS.run(self, input_lines, input_offset)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 239, in run
    context, state, transitions)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 460, in check_line
    return method(match, context, next_state)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2326, in explicit_markup
    nodelist, blank_finish = self.explicit_construct(match)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2338, in explicit_construct
    return method(self, expmatch)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2081, in directive
    directive_class, match, type_name, option_presets)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2130, in run_directive
    result = directive_instance.run()
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/directive.py", line 131, in run
    documenter.generate(more_content=self.content)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 819, in generate
    self.document_members(all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 740, in document_members
    check_module=members_check_module and not isattr)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 1247, in generate
    all_members=all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 819, in generate
    self.document_members(all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 1235, in document_members
    ModuleLevelDocumenter.document_members(self, all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 740, in document_members
    check_module=members_check_module and not isattr)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 816, in generate
    self.add_content(more_content)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 554, in add_content
    for i, line in enumerate(self.process_doc(docstrings)):
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 514, in process_doc
    self.options, docstringlines)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/application.py", line 510, in emit
    return self.events.emit(event, self, *args)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/events.py", line 80, in emit
    results.append(callback(*args))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 190, in process_docstring
    formatted_annotation = format_annotation(annotation)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 103, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 103, in <genexpr>
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 119, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
TypeError: 'NoneType' object is not iterable
```

Author: icgood

setup.cfg

@@ -30,7 +30,9 @@ install_requires =
     typing >= 3.5; python_version == "3.4"
 
 [options.extras_require]
-test = pytest >= 3.1.0
+test =
+    pytest >= 3.1.0
+    typing_extensions >= 3.5
 
 [flake8]
 max-line-length = 99

sphinx_autodoc_typehints.py

@@ -5,6 +5,11 @@
 from sphinx.util import logging
 from sphinx.util.inspect import Signature
 
+try:
+    from typing_extensions import Protocol
+except ImportError:
+    Protocol = None
+
 try:
     from inspect import unwrap
 except ImportError:
@@ -47,7 +52,8 @@ def format_annotation(annotation):
         if inspect.isclass(getattr(annotation, '__origin__', None)):
             annotation_cls = annotation.__origin__
             try:
-                if Generic in annotation_cls.mro():
+                mro = annotation_cls.mro()
+                if Generic in mro or (Protocol and Protocol in mro):
                     module = annotation_cls.__module__
             except TypeError:
                 pass  # annotation_cls was either the "type" object or typing.Type
@@ -116,10 +122,12 @@ def format_annotation(annotation):
             annotation_cls = annotation.__origin__
 
         extra = ''
-        if Generic in annotation_cls.mro():
+        mro = annotation_cls.mro()
+        if Generic in mro or (Protocol and Protocol in mro):
             params = (getattr(annotation, '__parameters__', None) or
                       getattr(annotation, '__args__', None))
-            extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
+            if params:
+                extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
 
         return ':py:class:`~{}.{}`{}'.format(annotation.__module__, annotation_cls.__qualname__,
                                              extra)

tests/test_sphinx_autodoc_typehints.py

@@ -5,6 +5,8 @@
 from typing import (
     Any, AnyStr, Callable, Dict, Generic, Mapping, Optional, Pattern, Tuple, TypeVar, Union, Type)
 
+from typing_extensions import Protocol
+
 from sphinx_autodoc_typehints import format_annotation, process_docstring
 
 T = TypeVar('T')
@@ -21,6 +23,14 @@ class B(Generic[T]):
     pass
 
 
+class C(Protocol):
+    pass
+
+
+class D(Protocol[T]):
+    pass
+
+
 class Slotted:
     __slots__ = ()
 
@@ -71,7 +81,10 @@ class Slotted:
     (Pattern[str],                  ':py:class:`~typing.Pattern`\\[:py:class:`str`]'),
     (A,                             ':py:class:`~%s.A`' % __name__),
     (B,                             ':py:class:`~%s.B`\\[\\~T]' % __name__),
-    (B[int],                        ':py:class:`~%s.B`\\[:py:class:`int`]' % __name__)
+    (B[int],                        ':py:class:`~%s.B`\\[:py:class:`int`]' % __name__),
+    (C,                             ':py:class:`~%s.C`' % __name__),
+    (D,                             ':py:class:`~%s.D`\\[\\~T]' % __name__),
+    (D[int],                        ':py:class:`~%s.D`\\[:py:class:`int`]' % __name__)
 ])
 def test_format_annotation(annotation, expected_result):
     result = format_annotation(annotation)