Exception occured while build help

Following callstack can be seen. It seems it has problems with Q_DECL_CONSTEXPR:

Exception occurred:
File "c:\program files (x86)\python38-32\lib\site-packages\sphinx\domains\cpp.py", line 6126, in _parse_type
raise self._make_multi_error(prevErrors, header) from exTyped
sphinx.util.cfamily.DefinitionError: Error when parsing function declaration.
If the function has no return type:
Error in declarator or parameters-and-qualifiers
Invalid C++ declaration: Expecting "(" in parameters-and-qualifiers. [error at 133]
template<typename SourceType, typename DestType, typename Rep = typename DestType::result_type, typename ValueType> Q_DECL_CONSTEXPR Rep convert (ValueType value)
-------------------------------------------------------------------------------------------------------------------------------------^
If the function has a return type:
Error in declarator or parameters-and-qualifiers
If pointer to member declarator:
Invalid C++ declaration: Expected '::' in pointer to member (function). [error at 137]
template<typename SourceType, typename DestType, typename Rep = typename DestType::result_type, typename ValueType> Q_DECL_CONSTEXPR Rep convert (ValueType value)
-----------------------------------------------------------------------------------------------------------------------------------------^
If declarator-id:
Invalid C++ declaration: Expecting "(" in parameters-and-qualifiers. [error at 137]
template<typename SourceType, typename DestType, typename Rep = typename DestType::result_type, typename ValueType> Q_DECL_CONSTEXPR Rep convert (ValueType value)
-----------------------------------------------------------------------------------------------------------------------------------------^
The full traceback has been saved in C:\Users\roebst2\AppData\Local\Temp\sphinx-err-k_qy8m52.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at https://github.com/sphinx-doc/sphinx/issues. Thanks!

Start in sphinx/domains/cpp.py at _parse_type and reproduce the help build using the reported declaration containing Q_DECL_CONSTEXPR. Trace why this declaration fails to parse, then verify that the build completes without the reported Sphinx exception.