Unless TraitB is an auto trait, that isn't currently valid?
From the reference:
> Trait objects are written as the optional keyword dyn followed by a set of trait bounds, but with the following restrictions on the trait bounds. All traits except the first trait must be auto traits, there may not be more than one lifetime, and opt-out bounds (e.g. ?Sized) are not allowed.
The only one of those restrictions that is acceptable to have is the single lifetime one. All the others are seriously restricting. The devs seem to agree, but work on this aspect of rust is very slow, and people are arguing on how to implement it. (I, for one, feel very strongly that dyn TraitA + TraitB should have a size of 3 pointers. That is, no magic combining vtables, just every added trait adds a new pointer to vtable.)
