error[E0277]: the size for values of type `<() as Lt<'_>>::T` cannot be known at compilation time
  --> $DIR/issue-53448.rs:12:54
   |
LL |     let f: &mut dyn FnMut<(_,), Output = ()> = &mut |_: <() as Lt<'_>>::T| {};
   |                                                      ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `<() as Lt<'_>>::T`
   = help: unsized fn params are gated as an unstable feature
help: consider further restricting the associated type
   |
LL | fn main() where <() as Lt<'_>>::T: Sized {
   |           ++++++++++++++++++++++++++++++
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL |     let f: &mut dyn FnMut<(_,), Output = ()> = &mut |_: &<() as Lt<'_>>::T| {};
   |                                                         +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.