[E0207] if type parameter used only in impl constraints

[KalitaAlexey]

use std::borrow::BorrowMut;

pub trait Value {
    fn get(&self) -> i32;
    fn set(&mut self, value: i32);
}

pub struct UnmanagedValue {
    value: i32,
}

impl Value for UnmanagedValue {
    fn get(&self) -> i32 {
        self.value
    }

    fn set(&mut self, value: i32) {
        self.value = value;
    }
}

pub struct ManagedValue {
    value: i32,
}

impl Value for ManagedValue {
    fn get(&self) -> i32 {
        self.value
    }

    fn set(&mut self, value: i32) {
        if self.value == value {
            return;
        }
        self.value = value;
    }
}

pub struct SetValueAction {
    value: i32,
}

pub trait Action<T>: Sized {
    fn execute(self, target: &mut T);
}

impl<V: Value, T: BorrowMut<V>> Action<T> for SetValueAction {
    fn execute(self, target: &mut T) {
        target.borrow_mut().set(self.value);
    }
}

fn main() {}

fails with error:

<anon>:47:6: 47:7 error: the type parameter `V` is not constrained by the impl trait, self type, or predicates [E0207]
<anon>:47 impl<V: Value, T: BorrowMut<V>> Action<T> for SetValueAction {
               ^
<anon>:47:6: 47:7 help: see the detailed explanation for E0207
error: aborting due to previous error
playpen: application terminated with error code 101

But this function compiles

pub fn execute<V: Value, T: BorrowMut<V>>(action: SetValueAction, target: &mut T) {
    target.borrow_mut().set(action.value);
}

[arielb1]

This is as per rust-lang/rfcs#447. <Action<Whatever> as SetValueAction>::execute must refer to a single method, but there can easily be multiple types that implement Value and are valid for BorrowMut.