[phobos] [D-Programming-Language/phobos] 601c75: Improve parsing error handling

[Don Clugston]

On 5 September 2011 09:35, Jonathan M Davis <jmdavisProg at gmx.com> wrote:
> On Monday, September 05, 2011 00:27:05 Walter Bright wrote:
>> On 9/4/2011 11:41 PM, Jonathan M Davis wrote:
>> > It still baffles me why anything other than a variable would be an
>> > lvalue
>>
>> *p is also an lvalue.
>
> Good point, but that's essentially the same as setting the return value of a
> function that returns by ref, which essentially is a variable. But I guess
> that it does come down to an address more than a variable. What I really don't
> get is the temporaries such as when you construct a struct in place - e.g.
> S(1) - functioning as lvalues. The result is that if you have an opEquals
> which takes const ref like it's supposed to (or any function which takes a
> const ref, but opEquals is a good example), then you can pass it S(1) but not
> a the result of a function that returns S(1).
>
> struct S
> {
>    bool opEquals(const ref S rhs)
>    {
>        return val == rhs.val;
>    }
>
>    int val;
> }
>
> S func()
> {
>    return S(1);
> }
>
> void main()
> {
>    auto s = S(1);
>    assert(s == S(1)); //compiles
>    assert(s == func()); //doesn't compile
> }
>
> Obviously, this is a complicated issue, but the situation above is downright
> confusing to many people (e.g.
> http://stackoverflow.com/questions/6986175/const-ref-and-rvalue-in-d ). It
> seems completely inconsistent. Even if C++ treats stuff as lvalues or not in
> exactly the same way in D, it's not an issue in the same way in C++, since
> const S& will take a temporary in C++, whereas const ref S won't take a
> temporary in D.

It's not merely confusing. AFAIK there is no possible signature of
opEquals which actually works.
I think two different issues may be being conflated here, though.
I think there's more of a problem with the semantics of 'const ref'
and 'auto ref', than with literals being lvalues.