[Issue 8408] Purity calculation should be improved

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8408



--- Comment #11 from Kenji Hara <k.hara.pg at gmail.com> 2012-12-10 19:39:00 PST ---
(In reply to comment #10)
> (In reply to comment #8)
> > (In reply to comment #6)
> > I'm not sure that this should be allowed.
> > ...
> 
> Why not? It is known at the call site that anything foo1 will return is newly
> allocated. Strong or weak purity is irrelevant.

Your argument had be true.

I had re-read issue 5081, and could be believed that between the purity level
(strong, constant, weak) and the conversion possibility to immutable of
returned value are irrelevant.

From: http://d.puremagic.com/issues/show_bug.cgi?id=5081#c2
> Note actually that as long as you can verify the return value did not come
> directly from the parameters, it's also possible to implicitly cast to
> immutable.
> 
> For example:
> 
> pure T[] mydup(T)(const(T)[] param) {...}
> 
> It's provable that the return value did not come from param (without a cast),
> because you can't implicitly cast param to T[].  So you can cast the result to
> immutable, >>>>even if param began as mutable<<<<.

The last sentence describes the ideal behavior.
Thanks a lot!

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