On 2013-11-14, at 6:20, Tommi <[email protected]> wrote:
> On 2013-11-14, at 5:54, Huon Wilson <[email protected]> wrote:
>
>> On 14/11/13 14:50, Tommi wrote:
>>> let mut n = 11;
>>> let p: &int = &123;
>>>
>>> p + n; // OK
>>> n + p; // error: mismatched types: expected `int` but found `&int`
>>>
>>> Shouldn't this error be considered a compiler-bug? The Add trait says '&'
>>> for rhs after all:
>>> fn add(&self, rhs: &RHS) -> Result;
>>>
>>> -Tommi
>>>
>>> _______________________________________________
>>> Rust-dev mailing list
>>> [email protected]
>>> https://mail.mozilla.org/listinfo/rust-dev
>>
>> This is https://github.com/mozilla/rust/issues/8895 . I believe it is caused
>> by the implementation details of the overloaded-operator desugaring (a + b
>> is equivalent to `a.add(&b)`, so auto-deref allows for `a: &T` with `b: T`),
>> I personally think the more sensible resolution would be for `p + n` to be
>> disallowed as well as `n + p`.
>>
>> Huon
>> _______________________________________________
>> Rust-dev mailing list
>> [email protected]
>> https://mail.mozilla.org/listinfo/rust-dev
>
>
> Thanks, I tried to search for a bug report for this but didn't manage to find
> that one. To me it makes perfect sense that p + n would work because if a + b
> is specified to be "just" syntactic sugar for a.add(b), then a + b should
> behave exactly like a.add(b), with implicit pointer dereferencing and all
> that good stuff. But, I don't understand why is it a.add(&b) like you said,
> though.
>
> By the way, I would hate all of this behavior if Rust had pointer arithmetic,
> but luckily it doesn't seem to have it.
>
> -Tommi
>
> _______________________________________________
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> [email protected]
> https://mail.mozilla.org/listinfo/rust-dev
You're right in that it's not a bug in built-in types, but affects user-defined
types also. But n.add(p) works just fine, while n.add(&p) doesn't:
struct Num { n: int }
impl Add<Num, Num> for Num {
fn add(&self, other: &Num) -> Num { Num { n: self.n + other.n } }
}
fn main() {
let n = 11i;
let p = &22i;
p + n; // OK
n + p; // error: mismatched types: expected `int` but found `&int`
n.add(p); // OK
n.add(&p); // error: mismatched types: expected `&int` but found `&&int`
let n = Num { n: 11 };
let p = &Num { n: 22 };
p + n; // OK
n + p; // error: mismatched types: expected `Num` but found `&Num`
n.add(p); // OK
n.add(&p); // error: mismatched types: expected `&Num` but found `&&Num`
}
-Tommi
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