Error code E0212

Cannot use the associated type of a trait with uninferred generic parameters.

Erroneous code example:

#![allow(unused)] fn main() { pub trait Foo<T> { type A; fn get(&self, t: T) -> Self::A; } fn foo2<I : for<'x> Foo<&'x isize>>( field: I::A) {} // error! }

In this example, we have to instantiate 'x, and we don't know what lifetime to instantiate it with. To fix this, spell out the precise lifetimes involved. Example:

#![allow(unused)] fn main() { pub trait Foo<T> { type A; fn get(&self, t: T) -> Self::A; } fn foo3<I : for<'x> Foo<&'x isize>>( x: <I as Foo<&isize>>::A) {} // ok! fn foo4<'a, I : for<'x> Foo<&'x isize>>( x: <I as Foo<&'a isize>>::A) {} // ok! }