Error code E0750

A negative impl was made default impl.

Erroneous code example:

#![feature(negative_impls)] #![feature(specialization)] trait MyTrait { type Foo; } default impl !MyTrait for u32 {} // error! fn main() {}

Negative impls cannot be default impls. A default impl supplies default values for the items within to be used by other impls, whereas a negative impl declares that there are no other impls. Combining it does not make sense.