Issue 9148 - 'pure' is broken

https://issues.dlang.org/show_bug.cgi?id=9148

A nested function inside `pure` function should also be typed as `pure`, and considered as weak purity function.
Actually, regardless of what's later in this function, why is this declaration even _legal_ in a `pure` function? It's not legal to _use_ it, since it's `static` and thus violates purity, so why even allow it to be declared in the first place?