Error code E0207

A type parameter that is specified for impl is not constrained.

Erroneous code example:

#![allow(unused)]
fn main() {
struct Foo;

impl<T: Default> Foo {
    // error: the type parameter `T` is not constrained by the impl trait, self
    // type, or predicates [E0207]
    fn get(&self) -> T {
        <T as Default>::default()
    }
}
}

Any type parameter of an impl must meet at least one of the following criteria:

• it appears in the implementing type of the impl, e.g. impl<T> Foo<T>
• for a trait impl, it appears in the implemented trait, e.g. impl<T> SomeTrait<T> for Foo
• it is bound as an associated type, e.g. impl<T, U> SomeTrait for T where T: AnotherTrait<AssocType=U>

Error example 1

Suppose we have a struct Foo and we would like to define some methods for it. The previous code example has a definition which leads to a compiler error:

The problem is that the parameter T does not appear in the implementing type (Foo) of the impl. In this case, we can fix the error by moving the type parameter from the impl to the method get:

#![allow(unused)]
fn main() {
struct Foo;

// Move the type parameter from the impl to the method
impl Foo {
    fn get<T: Default>(&self) -> T {
        <T as Default>::default()
    }
}
}

Error example 2

As another example, suppose we have a Maker trait and want to establish a type FooMaker that makes Foos:

#![allow(unused)]
fn main() {
trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker for FooMaker {
// error: the type parameter `T` is not constrained by the impl trait, self
// type, or predicates [E0207]
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}
}

This fails to compile because T does not appear in the trait or in the implementing type.

One way to work around this is to introduce a phantom type parameter into FooMaker, like so:

#![allow(unused)]
fn main() {
use std::marker::PhantomData;

trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

// Add a type parameter to `FooMaker`
struct FooMaker<T> {
    phantom: PhantomData<T>,
}

impl<T: Default> Maker for FooMaker<T> {
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo {
            foo: <T as Default>::default(),
        }
    }
}
}

Another way is to do away with the associated type in Maker and use an input type parameter instead:

#![allow(unused)]
fn main() {
// Use a type parameter instead of an associated type here
trait Maker<Item> {
    fn make(&mut self) -> Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker<Foo<T>> for FooMaker {
    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}
}

Additional information

For more information, please see RFC 447.