associated type does not satisfy its declared bound

Why this fires¶

An implementation selected a concrete associated type that does not satisfy a bound declared by the trait.

trait Source {
    type Item: Ord
}

struct Callback {}
impl Source for Callback {
    type Item = fn(int) -> int
    //   ^ error[TE480]: function does not implement Ord
}

The diagnostic names the implementation, the projected associated type, and the failed requirement. The original type is preserved; it is never replaced with Any merely to make the impl pass.

Fix it¶

Choose an associated type that implements every declared bound, or remove a bound from the trait only when consumers genuinely do not need that capability.

See also¶

• TE481 — required associated type is missing.
• TE482 — trait arguments select an incompatible witness.
• specs/gradual-type-precision.html.