pureity of closures

[Stefan Koch via Digitalmars-d]

On Friday, 27 March 2015 at 17:47:26 UTC, H. S. Teoh wrote:
> What I'm more concerned about is whether the current compiler
> implementation may accidentally allow leakage of the pure 
> function's
> internal context, which would break purity.
>
>
> T
please explain your reasoning with a bit of example code.
I am not sure if I get where/when impurity would be introduced.