> Pedantically the correct way is to cast to a function pointer with no prototype (empty parentheses)

Thanks for raising this point Antti. This is hinted at by the gcc 8 documentation on -Wcast-function-type: "The function type void (*) (void) is special and matches everything, which can be used to suppress this warning" [1]. One cannot use empty parentheses though as this raises -Wstrict-prototypes on Python builds with gcc and -Wcast-function-type with gcc 8.

So (void *) may be replaced with (void (*) (void)) in my previous code snippets and also in PR 6748 and PR 6749 (I just checked my code).

[1] https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#Warning-Options