Warn against wrong class opEquals signature usage

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2013-06-07T14:42:07Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=10292

Description

class Foo {
    int x;
    this(int x) {
        this.x = x;
    }
    bool opEquals(in Foo a) const {
        return a.x == this.x;
    }
}
void main() {
    Foo f1 = new Foo(10);
    Foo f2 = new Foo(10);
    assert(f1 != f2);
    assert(f1.opEquals(f2));
}


/*
This code compiles with no errors nor warnings with dmd 2.064alpha.

Expected something like:

test(6): Warning. Class opEquals should have signature like: override bool opEquals(Object) const


Here a little more correct opEquals is:

    override bool opEquals(Object a) const {
        auto fooa = cast(Foo)a;
        if (fooa is null)
            return false;
        return fooa.x == this.x;
    }


The compiler needs to give warnings or errors for such cases of wrong signatures. If you don't write opEquals(Object) it needs to complain. I think the current situation of silent accepting wrong/useless special methods is not acceptable in modern language.

(Code adapted from a post by "Namespace" in D.learn).