trait type arguments are incompatible with the required witness

Why this fires¶

The concrete type implements the requested trait name, but with different type arguments. Trait witnesses are exact: implementing Sink<str> does not prove Sink<int>.

trait Sink<T> { fn push(self, value: T) }
struct Log {}
impl Sink<str> for Log { fn push(self, value: str) {} }

fn write_int<S>(sink: S) where S: Sink<int> {}
write_int(Log {})
//        ^ error[TE482]: found Sink<str>, required Sink<int>

Fix it¶

Pass a type with the exact required witness, change the generic bound, or add a non-overlapping implementation for the required arguments when the semantics are valid.

See also¶

• TE820 — trait is not implemented at all.
• TE480 — associated type bound failure.
• specs/gradual-type-precision.html.