warning: default bound relaxed for a type parameter, but this does nothing because the given bound is not a default; only `?Sized` is supported
  --> $DIR/issue-87199.rs:8:8
   |
LL | fn arg<T: ?Send>(_: T) {}
   |        ^

warning: default bound relaxed for a type parameter, but this does nothing because the given bound is not a default; only `?Sized` is supported
  --> $DIR/issue-87199.rs:10:12
   |
LL | fn ref_arg<T: ?Send>(_: &T) {}
   |            ^

warning: default bound relaxed for a type parameter, but this does nothing because the given bound is not a default; only `?Sized` is supported
  --> $DIR/issue-87199.rs:12:13
   |
LL | fn ret() -> impl Iterator<Item = ()> + ?Send { std::iter::empty() }
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
  --> $DIR/issue-87199.rs:18:22
   |
LL |     ref_arg::<[i32]>(&[5]);
   |     ---------------- ^^^^ doesn't have a size known at compile-time
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Sized` is not implemented for `[i32]`
note: required by a bound in `ref_arg`
  --> $DIR/issue-87199.rs:10:12
   |
LL | fn ref_arg<T: ?Send>(_: &T) {}
   |            ^ required by this bound in `ref_arg`
help: consider relaxing the implicit `Sized` restriction
   |
LL | fn ref_arg<T: ?Send + ?Sized>(_: &T) {}
   |                     ++++++++

error: aborting due to previous error; 3 warnings emitted

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