[Issue 8161] give an error for invalid property functions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=8161

--- Comment #13 from anonymous4 <dfj1esp02 at sneakemail.com> ---
(In reply to bitwise from comment #11)
> enum Accessibility { Read, Write, ReadWrite }
> 
> template isUseableAsFieldOf(T, F, Accessibility access =
> Accessibility.Both)(F fun) {...}

std.traits should provide this template irrespective of what happens to
properties, because currently this idiom exists in the language, but not
supported by reflection facilities.

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