E0016 — Generic trait bound not satisfied

A type argument doesn’t implement a bound required by the generic function or struct it’s substituted into.

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What triggers it

A generic function or struct declares a trait bound on one of its type parameters (e.g. fn max<T: Ord>(...)). Instantiating it with a concrete type that has no matching impl fails this check.

Minimal example

trait Ord { fn cmp(self, Self other) -> int; }
fn max<T: Ord>(T a, T b) -> T { return a; }
struct Point { int x; int y; }
fn main() {
    max(Point { x: 1, y: 2 }, Point { x: 3, y: 4 });
}

Output:

scratch.rz:5:5: error[E0016]: `Point` does not implement `Ord`
    max(Point { x: 1, y: 2 }, Point { x: 3, y: 4 });
    ^^^

Fix

Provide an impl Ord for Point { ... }, or call the generic function with a type that already satisfies the bound.

Source

Emitted from the typechecker’s generic-instantiation check in resilient/src/typechecker.rs.