Unexpected conflicting implementation for TryFrom

Question

I want to implement the trait TryFrom in a case like this:

struct T1;
struct T2(T1);

impl<U> TryFrom<U> for T2 where U: Into<T1> {
    type Error = ();
    fn try_from(val:U) -> Result<T2, Self::Error> {
        unimplemented!();
    }
}

which gives me this error:

error[E0119]: conflicting implementations of trait `std::convert::TryFrom<_>` for type `T2`
 --> src/lib.rs:4:1
  |
4 | impl<U> TryFrom<U> for T2 where U: Into<T1> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl<T, U> TryFrom<U> for T
            where U: Into<T>;

Here U implement Into<T1> not Into<T2> thus I don't really understand the error. I tried to confirm there is no blanket implementation by implementing this and it compiles without conflict:

struct T1;
struct T2(T1);

impl Into<T2> for T1 {
    fn into(self) -> T2 {
        unimplemented!();
    }
}

Is there any way around it? I don't want a default implementation here.

Answer 1

Trait implementations are not allowed to conflict. A type is perfectly allowed to implement both Into<T1> and Into<T2> . Your implementation would cause an ambiguity in this case whether to use the T: TryFrom<U> implementation by the built-in blanket implementation or from your blanket implementation. And thus there is a conflict and the compiler rejects your implementation.

Essentially you cannot implement TryFrom generically because of the blanket implementation; you can't avoid a potential conflict.

Your confirmation doesn't mean anything since there is no such blanket implementation for Into that would conflict between T1 and T2 . If you tried to implement it generically , like impl Into<T2> for U where U: Into<T1> , that'd be a problem as well. The genericism is the core problem.