Spurious error with function pointer as template argument

[wintersteiger]

Describe the bug

For this:

/// @brief A template class
template <void (*F)()>
class C {};

Sphinx complains thusly:

| [(DefinitionError('Invalid C++ declaration: Expected "," or ">". [error at 19]\n  template<void(*)() F> C\n  -------------------^'), 'If no parameter')]
|/data/cwinter/breathe/source/index.rst:11: WARNING: Error in template parameter list.
Invalid C++ declaration: Expected "," or ">". [error at 19]
  template<void(*)() F> C
  -------------------^

To Reproduce

Copy source snippet above into .h file and run Sphinx with Breathe extension enabled (see project files)

$ sphinx-autobuild source build

Expected behavior

Don't throw an error and produce the right type names. (When the error is thrown some types are assigned names with PhonyNameDueToError.)

Your project

sphinx-templates.zip

Environment info

• OS: Ubuntu 18.04
• Python version: 3.8.0
• Sphinx version: 3.5.1
• Sphinx extensions: breathe
• Extra tools: doxygen 1.9.2

(From sphinx-doc/sphinx#8953)

[jakobandersen]

This is quite an icky bug. The XML for C is

<compounddef id="classC" kind="class" language="C++" prot="public">
  <compoundname>C</compoundname>
  <includes refid="test_8hpp" local="no">test.hpp</includes>
  <templateparamlist>
    <param>
      <type>void(*)()</type>
      <declname>F</declname>
      <defname>F</defname>
    </param>
  </templateparamlist>
  ...
</compounddef>

I don't see a generic way of getting the parameter name F in the correct part of the type void (*)() without parsing the type, in the general case.
I suggest using the Sphinx C++ parser to get the AST to assemble the paramter the right way.

[wintersteiger]

Yes, I think it's impossible to do this without parsing the type. I'm not familiar with the Sphinx C++ parser - is that in some way configurable by the end-user or would this require changes to the parser implementation?