Operator requires a trait bound on the type parameter

Why this fires¶

An operator is used on a generic type parameter that doesn't carry the corresponding trait bound. Without the bound the compiler can't guarantee the operation is valid for every possible type argument.

fn add_two<T>(a: T, b: T) -> T {
    return a + b
    //     ^^^^^ error[TE821]: operator `+` requires bound `Add` on `T`
}

Fix it¶

Add the missing bound in either the angle-bracket or where form:

// Angle-bracket form
fn add_two<T: Add>(a: T, b: T) -> T {
    return a + b
}

// where form (useful when multiple bounds are needed)
fn add_two<T>(a: T, b: T) -> T where T: Add {
    return a + b
}

Operator → bound mapping¶

Only binary operators are gated. Unary - on a generic parameter is not checked yet (its Neg bound is recognised but unenforced).

See also¶

• TE820 — the concrete type passed to a bounded generic doesn't implement the required trait.
• TE822 — bound names an unknown trait.