Memory allocation purity

[via Digitalmars-d]

On Thursday, 15 May 2014 at 12:57:43 UTC, Steven Schveighoffer 
wrote:
> To be honest, code that would exploit such an anomaly is only 
> ever used in "proof" exercises, and never in real code. I don't 
> think it's an issue.

That's the wrong attitude to take when it comes to the compiler 
and runtime.

If it verifies, it should verify. Not mostly, but fully, 
otherwise "weakly pure" becomes a programmer guarantee. Which is 
ok too, but either the one or the other.

How can you prove that such issues don't arise in real code?