zig/doc/langref/test_inferred_error_sets.zig

// With an inferred error set
pub fn add_inferred(comptime T: type, a: T, b: T) !T {
    const ov = @addWithOverflow(a, b);
    if (ov[1] != 0) return error.Overflow;
    return ov[0];
}

// With an explicit error set
pub fn add_explicit(comptime T: type, a: T, b: T) Error!T {
    const ov = @addWithOverflow(a, b);
    if (ov[1] != 0) return error.Overflow;
    return ov[0];
}

const Error = error {
    Overflow,
};

const std = @import("std");

test "inferred error set" {
    if (add_inferred(u8, 255, 1)) |_| unreachable else |err| switch (err) {
        error.Overflow => {}, // ok
    }
}

// test