pub type PFNGLGETDOUBLEI_VPROC = Option<unsafe extern "C" fn(target: GLenum, index: GLuint, data: *mut GLdouble)>;
enum PFNGLGETDOUBLEI_VPROC { None, Some(unsafe extern "C" fn(_: u32, _: u32, _: *mut f64)), }
No value.
Some value of type T.
T