This option enables or disables the recognition of guiding declarations for template functions in C++.
A guiding declaration is a function declaration that matches an instance of a function template but has no explicit definition because its definition derives from the function template.
--no_guiding_decls is combined with
a specialization of a nonmember template function is not recognized.
It is treated as a definition of an independent function.
--guiding_decls is provided only
as a migration aid for legacy source code that does not conform
to the C++ standard. Its use is not recommended.