error[E0308]: mismatched types --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:20:43 | LL | let _: fn(&mut &isize, &mut &isize) = a; | ---------------------------- ^ one type is more general than the other | | | expected due to this | = note: expected fn pointer `for<'a, 'b, 'c, 'd> fn(&'a mut &'b _, &'c mut &'d _)` found fn item `for<'a, 'b> fn(&'a mut &_, &'b mut &_) {a::<'_, '_>}` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0308`.