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

[Jonathan M Davis]

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.

- Jonathan M Davis