undeclared or overlapping trait impl member

Why this fires¶

An implementation is not coherent with its trait declaration. Either it adds a method the trait does not declare, or another implementation can satisfy the same parameterized target and trait instance.

trait Convert<T> { fn convert(self, value: T) -> T }
struct Converter {}

impl Convert<int> for Converter {
    fn convert(self, value: int) -> int { value }
}
impl Convert<int> for Converter {
//   ^^^^^^^^^^^^ error[TE303]: overlapping impl
    fn convert(self, value: int) -> int { value }
}

Fix it¶

Keep exactly one witness for each target pattern and exact trait instance. Different instances such as Convert<int> and Convert<str> may coexist when their arguments cannot overlap. Remove methods that are not part of the trait, or move them to an inherent impl Type block.

See also¶

• TE304 — an implemented method has the wrong signature.
• TE305 — an associated type contract is invalid.