[Issue 9238] Support rvalue references

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9238



--- Comment #8 from Kenji Hara <k.hara.pg at gmail.com> 2012-12-29 23:39:37 PST ---
(In reply to comment #7)
> And what's the problem with leaving auto ref as it is with templated functions

In recent, I have used `auto ref` in std.algorithm.forward.

Yes, ideally we can remove current 'templated auto ref'. There is an
alternative solution (Make two overloaded functions - one receives rvalue, and
the other receives lvalue -, and then @disable either one). But it is a
breaking change, as you say "It subverts what in currently does".

`in ref` is very recently allowed from 2.060. `auto ref` is from 2.038.
Then, removing current `auto ref` is much impact than changing `in ref`
meaning.

> rvalues already work just fine with auto ref and templated functions. It's just
> non-templated functions which lack a solution.

> Once scope has actually been fixed to actually check for escaping references, you'll either end up with conflicting behavior with in depending on whether it's ref or not, or you'll end up with scope's restrictions on it, which would be horrendously over-restrictive. 

At least it is a necessary restriction for `in ref`. For example, we should not
allow following code.

ref T foo(in ref T t) { return t; }

If foo _actually_ receives a lvalue, returning t by ref is valid. But, if foo
receives an rvalue, foo accidentally returns a dangling reference, and it's
completely unsafe. So, we must select a conservative way at the point.

---

Here, I want to double-check the feature which is discussed.

Current `auto ref` with template function makes one or more template instances
based on the actual argument lvalue-ness. It might cause template bloating, and
for big size rvalue, object bit-copy is inefficient.

On the other hand, the discussed feature in the forum is as like "const T& in
C++". It can bind both lvalue and rvalue, and it will be passed to function via
"reference" (e.g. pointer). And, it works with non-template functions, and
template instantiation is not related.

Right?

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