An unnecessary type parameter was given in a type alias.
Erroneous code example:
type Foo<T> = u32; // error: type parameter `T` is never used
// or:
type Foo<A, B> = Box<A>; // error: type parameter `B` is never used
Please check you didn't write too many parameters. Example:
type Foo = u32; // ok!
type Foo2<A> = Box<A>; // ok!