//@ check-pass trait Variable<'a> { type Type; } impl Variable<'_> for () { type Type = (); } fn check(_: F) where F: Fn(T), // <- if removed, all fn_* then require type annotations F: for<'a> Fn(>::Type), T: for<'a> Variable<'a>, { } fn test(arg: impl Fn(())) { fn fn_1(_: ()) {} let fn_2 = |_: ()| (); let fn_3 = |a| fn_1(a); let fn_4 = arg; check(fn_1); // Error check(fn_2); // Ok check(fn_3); // Ok check(fn_4); // Error } fn main() {}