[Issue 8161] give an error for invalid property functions

[d-bugmail at puremagic.com]

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

--- Comment #12 from Jonathan M Davis <issues.dlang at jmdavisProg.com> ---
I'd advise against relying on @property for any kind of metaprogramming at this
point given that it doesn't actually have anything to do with whether the
function can be or is used as a property and that its future in the language is
uncertain.

I don't think that there's any question that we _could_ do something more with
@property, and it may yet be salvaged to do something useful related to
properties, but whatever that may be really needs to be designed and then
gotten past Walter and Andrei - and that means a DIP. Certainly, the idea that
@property defines what can and can't be used as a property function is dead at
this point.

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