struct MyErr: ErrorType {}

func foo<T>(@noescape f: () throws -> T) throws -> T {
    throw MyErr()
}

func bar<T,U>(x: T, @noescape f: T throws -> U) rethrows -> U {
    // if we called
    //   return try foo({ try f(x) })
    // the compiler would yell at us for calling a `throws` function.
    // But if we wrap it in `idemp()` the compiler doesn't yell at us
    // even though it should.
    return try idemp(x, f: { x in try foo({ try f(x) }) })
}

func idemp<T,U>(x: T, @noescape f: T throws -> U) rethrows -> U {
    return try f(x)
}

// As a consequence, because foo() unconditionally throws, the following
// results in the program aborting execution (without even an error message).
print(bar(1, f: { $0+1 }))