`&5i + 5i` compiles but `5i + &5i` or `&5i == 5i` don't

[ben0x539]

Auto-borrowing or auto-dereferencing are confusing me here, but this feels somewhat weird and inconsistent.

I wonder why all the operator overloads need to implemented methods rather than associatied functions where all the operands could be equally unsurprising arguments and not half arguments, half method receivers, but I'm not sure how applicable that is to this nitpick.

[nikomatsakis]

Related to #4920. My proposal is to make == and other operators never do any form of auto-coercion.

I am also inclined to agree associated fns would be better than methods but it doesn't really make any difference to the question at hand, which is what the behavior should be when used as binary operators.

[flaper87]

Visiting for triage, still needs to be fixed.

Proposed bug example:

fn main() {
    &5i + 5i;
    5i + &5i;
    &5i == 5i;
}

[pnkfelix]

cc me

[ben0x539]

oh hey this went away with the introduction of by-value operator overloading traits