[Issue 3572] declaring pure function with void return type should be compile time error

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=3572

--- Comment #10 from Michal Minich <michal.minich at gmail.com> ---
Agreed. even 

pure void nothrow never() { assert(false); } 

can throw Error, which is observable effect.

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