Binding associated type requires redundancy.

[frankmcsherry]

I've been told this may be intended behavior, in which case treat this as a vote for "I think it shouldn't be". :)

If you have a trait with a constraint on an associated type,

pub trait Test {
    type Item;
    type Bundle: From<Self::Item>;
}

You can use the trait without constraints, like so

fn works<T>()
    where T: Test,
{ }

but if you instantiate any associated types it breaks.

fn fails<T>() 
    where T: Test<Item = String>
{ } 
// error[E0277]: the trait bound `<T as Test>::Bundle: std::convert::From<std::string::String>` is not satisfied
// ..
//   = help: consider adding a `where <T as Test>::Bundle: std::convert::From<std::string::String>` bound

You can solve the problem by copy/paste with the trait constraint.

fn works<T>()
    where T: Test<Item = String>,
          T::Bundle: From<T::Item>,
{ }

I've been pointed at 28055, related to super traits (same, maybe not?). I think the invoked argument is that "too many inferred constraints leads to spooky action at a distance", which I understand.

At the moment, I very much feel like the maintainability trade-off is in totally the wrong direction, though. If this associated type changes, or goes away, or has a new constraint, all existing code that may not care about the type breaks until you copy/paste in the (I feel) redundant requirements the trait already imposes.

Edit: see also #34106.

[estebank]

Currently we suggest the appropriate type constraint:

error[E0277]: the trait bound `<T as Test>::Bundle: std::convert::From<std::string::String>` is not satisfied
  --> src/lib.rs:10:1
   |
1  |   pub trait Test {
   |   -------------- required by `Test`
...
10 | / fn fails<T>() 
11 | |     where T: Test<Item = String>
   | |                                 - help: consider further restricting the associated type: `, <T as Test>::Bundle: std::convert::From<std::string::String>`
12 | | { } 
   | |___^ the trait `std::convert::From<std::string::String>` is not implemented for `<T as Test>::Bundle`

There are multiple other tickets to make the code work, but now at least we direct people in the right direction.