error[E0277]: `F` cannot be sent between threads safely
  --> $DIR/closure-bounds-cant-promote-superkind-in-struct.rs:15:1
   |
LL | / fn foo<F>(blk: F) -> X<F> where F: FnOnce() + 'static {
LL | |     //~^ ERROR `F` cannot be sent between threads safely
LL | |     return X { field: blk };
LL | | }
   | |_^ `F` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `F`
   = help: consider adding a `where F: std::marker::Send` bound
note: required by `X`
  --> $DIR/closure-bounds-cant-promote-superkind-in-struct.rs:11:1
   |
LL | struct X<F> where F: FnOnce() + 'static + Send {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

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