sphinx.util.cfamily.DefinitionError: Error in template parameter list.

[ezyang]

Describe the bug
I appear to have some valid C++14 syntax which causes Sphinx's C++ parser to choke. Here is the full error trace:

# Sphinx version: 3.1.2
# Python version: 3.7.4 (CPython)
# Docutils version: 0.16 release
# Jinja2 version: 2.11.2
# Last messages:
#   reading sources... [ 36%] api/function_namespacec10_1_1detail_1a0ae4a0b76b5203a055ad8e184734a642
#   reading sources... [ 36%] api/function_namespacec10_1_1detail_1a38bac4f70cbe3e60b11e84a4af793018
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1a63a885be061e64e85482d1bc657d9eab
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1a7fd879319c4690483b10f7377cdc0c14
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1aa26558d64e565f26516bd99850b6df8f
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1aba4bf15cd6c6b43bb04a70f67dd5dd98
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1ae289a3f407b2fe02ae5012a59b453f8f
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1af0966c5849fbf09f16675056a0bd7bda
#   reading sources... [ 37%] api/function_namespacec10_1_1detail_1af8326a119322305e6244a45e6acd2ecc
#   reading sources... [ 37%] api/function_namespacec10_1_1detail___1a8657ebe87695069950896d52bf3504ca
# Loaded extensions:
#   sphinx.ext.mathjax (3.1.2) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/ext/mathjax.py
#   sphinxcontrib.applehelp (1.0.2) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinxcontrib/applehelp/__init__.py
#   sphinxcontrib.devhelp (1.0.2) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinxcontrib/devhelp/__init__.py
#   sphinxcontrib.htmlhelp (1.0.3) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinxcontrib/htmlhelp/__init__.py
#   sphinxcontrib.serializinghtml (1.1.4) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinxcontrib/serializinghtml/__init__.py
#   sphinxcontrib.qthelp (1.0.3) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinxcontrib/qthelp/__init__.py
#   alabaster (0.7.12) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/alabaster/__init__.py
#   sphinx.ext.intersphinx (3.1.2) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/ext/intersphinx.py
#   breathe (4.19.2) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/breathe/__init__.py
#   exhale (0.2.3) from /Users/ezyang/miniconda3/lib/python3.7/site-packages/exhale/__init__.py
#   pytorch_sphinx_theme (unknown version) from /Users/ezyang/Dev/pytorch-tmp/docs/cpp/src/pytorch-sphinx-theme/pytorch_sphinx_theme/__init__.py
Traceback (most recent call last):
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/cmd/build.py", line 280, in build_main
    app.build(args.force_all, filenames)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/application.py", line 348, in build
    self.builder.build_update()
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 299, in build_update
    len(to_build))
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 311, in build
    updated_docnames = set(self.read())
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 418, in read
    self._read_serial(docnames)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 439, in _read_serial
    self.read_doc(docname)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 479, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/io.py", line 221, in read_doc
    pub.publish()
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/core.py", line 218, in publish
    self.settings)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/io.py", line 126, in read
    self.parse()
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/readers/__init__.py", line 77, in parse
    self.parser.parse(self.input, document)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/parsers.py", line 102, in parse
    self.statemachine.run(inputlines, document, inliner=self.inliner)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 171, in run
    input_source=document['source'])
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 242, in run
    context, state, transitions)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 459, in check_line
    return method(match, context, next_state)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2769, in underline
    self.section(title, source, style, lineno - 1, messages)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 327, in section
    self.new_subsection(title, lineno, messages)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 395, in new_subsection
    node=section_node, match_titles=True)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 282, in nested_parse
    node=node, match_titles=match_titles)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 196, in run
    results = StateMachineWS.run(self, input_lines, input_offset)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 242, in run
    context, state, transitions)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 459, in check_line
    return method(match, context, next_state)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2769, in underline
    self.section(title, source, style, lineno - 1, messages)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 327, in section
    self.new_subsection(title, lineno, messages)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 395, in new_subsection
    node=section_node, match_titles=True)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 282, in nested_parse
    node=node, match_titles=match_titles)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 196, in run
    results = StateMachineWS.run(self, input_lines, input_offset)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 242, in run
    context, state, transitions)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/statemachine.py", line 459, in check_line
    return method(match, context, next_state)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2342, in explicit_markup
    nodelist, blank_finish = self.explicit_construct(match)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2354, in explicit_construct
    return method(self, expmatch)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2097, in directive
    directive_class, match, type_name, option_presets)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2146, in run_directive
    result = directive_instance.run()
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/breathe/directives.py", line 99, in run
    node_stack = self.resolve_function(matches, args, project_info)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/breathe/directives.py", line 239, in resolve_function
    mask_factory, directive_args)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/breathe/directives.py", line 212, in create_function_signature
    ast = parser.parse_declaration('function', 'function')
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/domains/cpp.py", line 6492, in parse_declaration
    templatePrefix = self._parse_template_declaration_prefix(objectType)
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/domains/cpp.py", line 6418, in _parse_template_declaration_prefix
    raise e
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/domains/cpp.py", line 6413, in _parse_template_declaration_prefix
    params = self._parse_template_parameter_list()
  File "/Users/ezyang/miniconda3/lib/python3.7/site-packages/sphinx/domains/cpp.py", line 6310, in _parse_template_parameter_list
    raise self._make_multi_error(prevErrors, header)
sphinx.util.cfamily.DefinitionError: Error in template parameter list.
Invalid C++ declaration: Expected "=", ",", or ">". [error at 33]
  template<typename T, typename std::enable_if< !has_overloaded_addressof< T > ::value, bool >::type = false> constexpr T * static_addressof (T &ref)
  ---------------------------------^

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/ezyang/pytorch -b poc/sphinx-failure
$ cd pytorch/docs/cpp
$ pip install -r requirements.txt
$ make html

Expected behavior
Build should succeed

[jakobandersen]

Thanks for reporting. I believe it's because the parser assumes that every template parameter that starts with typename must be a type parameter. Your example clearly illustrates this is not always the case.