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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Is the type parameter `O` unconstrained?

Consider this example:

trait MyFuture<Arg> {
    //type Output: Future;
}
impl< F: Fn(Arg) -> O,Arg:Send, O: Future<Output = U>, U> MyFuture<Arg> for F {
    //type Output = O;
}

The type parameter O is neither in the impl trait, self type, nor predicates. However, this example can be compiled. If you remove Arg from MyFuture, for example:

trait MyFuture {
    //type Output: Future;
}
impl< F: Fn(Arg) -> O,Arg:Send, O: Future<Output = U>, U> MyFuture for F {
    //type Output = O;
}

The compiler will report an error:

   Compiling playground v0.0.1 (/playground)
error[E0207]: the type parameter `Arg` is not constrained by the impl trait, self type, or predicates
 --> src/main.rs:4:23
  |
4 | impl< F: Fn(Arg) -> O,Arg:Send, O: Future<Output = U>, U> MyFuture for F {
  |                       ^^^ unconstrained type parameter

error[E0207]: the type parameter `O` is not constrained by the impl trait, self type, or predicates
 --> src/main.rs:4:33
  |
4 | impl< F: Fn(Arg) -> O,Arg:Send, O: Future<Output = U>, U> MyFuture for F {
  |                                 ^ unconstrained type parameter

error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
 --> src/main.rs:4:56
  |
4 | impl< F: Fn(Arg) -> O,Arg:Send, O: Future<Output = U>, U> MyFuture for F {
  |                                                        ^ unconstrained type parameter

which implies that both O and U are unconstrained type parameters. What's the reason here?

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