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Error code E0091

An unnecessary type parameter was given in a type alias.

Erroneous code example:

#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { type Foo = u32; // ok! type Foo2<A> = Box<A>; // ok! }