pub type PFNGLGETINVARIANTBOOLEANVEXTPROC = Option<unsafe extern "C" fn(id: GLuint, value: GLenum, data: *mut GLboolean)>;
enum PFNGLGETINVARIANTBOOLEANVEXTPROC { None, Some(unsafe extern "C" fn(_: u32, _: u32, _: *mut u8)), }
No value.
Some value of type T.
T