error: vtable entries for `<S as B>`: [
           MetadataDropInPlace,
           MetadataSize,
           MetadataAlign,
           Method(<S as A>::foo_a1),
           Vacant,
           Method(<S as B>::foo_b1),
           Vacant,
       ]
  --> $DIR/vtable-vacant.rs:15:1
   |
LL | / trait B: A {
LL | |
LL | |     fn foo_b1(&self) {}
LL | |     fn foo_b2(&self) where Self: Send {}
LL | | }
   | |_^

error: aborting due to previous error