Basicly I want to write pure functions that will throw a compile-time error when they are not pure.

Your definition of 'pure' is different from my definition of 'pure'.

A 'pure' (as in functional programming) function ~can~ use external variables and still be called pure. It must not modify these variables, but it would be perfectly fine to have a sin(x) where you provide π as an external value. Changing π will change the actual output value, but if you repeat the calculation it will a) result in the same output value and b) x and π will not be modified.