The rb_define_method function takes a pointer to ANYARGS-ed functions,
which in fact varies 18 different prototypes. We still need to
preserve ANYARGS for storages but why not check the consistencies if
possible.
Q&As:
Q: Where did the magic number "18" came from in the description above?
A: Count the case branch of vm_method.c:call_cfunc_invoker_func().
Note also that the 18 branches has lasted for at least 25 years.
See also 200e0ee2fd.
Q: What is this __weakref__ thing?
A: That is a kind of function overloading mechanism that GCC provides.
In this case for instance rb_define_method0 is an alias of
rb_define_method, with a strong type.
Q: What is this __transparent_union__ thing?
A: That is another kind of function overloading mechanism that GCC
provides. In this case the attributed function pointer is either
VALUE(*)(int,VALUE*,VALUE) or VALUE(*)(int,const VALUE*,VALUE).
This is better than void* or ANYARGS because we can reject all
other possibilities than the two.
Q: What does this rb_define_method macro mean?
A: It selects appropriate alias of the rb_define_method function,
depending on the arity.
Q: Why the prototype change of rb_f_notimplement?
A: Function pointer to rb_f_notimplement is special cased in
vm_method.c:rb_add_method_cfunc(). That should be handled by the
__builtin_choose_expr chain inside of rb_define_method macro
expansion. In order to do so, comparison like (func ==
rb_f_notimplement) is inappropriate for __builtin_choose_expr's
expression (which must be a compile-time integer constant but the
address of rb_f_notimplement is not fixed until the linker). So
instead we are using __builtin_types_compatible_p, and in doing so
we need to distinguish rb_f_notimplement from others, by type.
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