'Auto can only be used for template function arguments' what?

[Jonathan M Davis]

On Wednesday, June 27, 2012 13:33:10 Timon Gehr wrote:
> On 06/27/2012 01:12 PM, kenji hara wrote:
> > 2012/6/27 Timon Gehr<timon.gehr at gmx.ch>:
> >> On 06/27/2012 07:27 AM, kenji hara wrote:
> >>> 2012/6/27 Jonathan M Davis<jmdavisProg at gmx.com>:
> >>>> On Wednesday, June 27, 2012 07:02:28 Mehrdad wrote:
> >>>>> On Wednesday, 27 June 2012 at 04:26:08 UTC, Jonathan M Davis
> >>>>> 
> >>>>> wrote:
> >>>>>> And how would that work? auto ref duplicates the function. If
> >>>>>> you pass it an lvalue, then it generates a ref version.
> >>>>> 
> >>>>> Two solutions:
> >>>>> 
> >>>>> - Turn 'auto ref' into 'ref', but simply have the compiler
> >>>>> generate a copy for the caller if he wants to pass by value?
> >>>> 
> >>>> That at least sounds like it would work. Walter may have a reason why
> >>>> it
> >>>> doesn't though, since he's the one that said that he didn't think that
> >>>> it
> >>>> was
> >>>> possible. Maybe he just didn't think of it, or maybe it causes some
> >>>> other
> >>>> problem that I can't think of.
> >>> 
> >>> After considering about 'auto ref', I was concluded that is an
> >>> inconsistency of current language spec and we cannot fix it correctly.
> >>> 
> >>> http://d.puremagic.com/issues/show_bug.cgi?id=8204
> >>> 
> >>> Therefore, I have created a thread to suggest new 'auto ref' recently.
> >>> 
> >>> http://forum.dlang.org/thread/CAFDvkcvf6G8Mc01Tds6ydXqCZbfp1q-a-oeFVk6BG
> >>> EtwCiUAqg at mail.gmail.com
> >>> 
> >>> Kenji Hara
> >> 
> >> How does it deal with this case?
> >> 
> >> auto ref id(auto ref arg){ return arg; }
> > 
> > In this case, arg might bind rvalue allocated in the caller of id, so
> > returning it with ref should be rejected statically.
> > But it is yet not implemented in pull#1019.
> > 
> > Kenji Hara
> 
> This is a quite severe limitation compared to how auto ref works today.
> As the return ref-ness is inferred and therefore the source code must
> be available, wouldn't it be a better option to decide
> lvalue/rvalue-ness at the call site? The compiler would just have to
> examine the first return statement in the function body.

Considering that if you said that a parameter is auto ref, you're basically 
saying that it's being passed by value except that you'd like the compiler to 
avoid the copy if it can, I don't see why it's restrictive at all to make 
returning a ref to the parameter illegal. It would be like if you wanted to 
return a & to a const& parameter in C++, which would be a very bad idea.

- Jonathan M Davis