zig/doc/langref/test_coerce_error_subset_to_superset.zig

const std = @import("std");

const FileOpenError = error{
    AccessDenied,
    OutOfMemory,
    FileNotFound,
};

const AllocationError = error{
    OutOfMemory,
};

test "coerce subset to superset" {
    const err = foo(AllocationError.OutOfMemory);
    try std.testing.expectEqual(FileOpenError.OutOfMemory, err);
}

fn foo(err: AllocationError) FileOpenError {
    return err;
}

// test