error: `C` is object-safe and has multiple supertraits
  --> $DIR/multiple_supertrait_upcastable.rs:7:1
   |
LL | trait C: A + B {}
   | ^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> $DIR/multiple_supertrait_upcastable.rs:2:9
   |
LL | #![deny(multiple_supertrait_upcastable)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error