[Issue 11161] Document the default struct equality comparison and operator overloading

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http://d.puremagic.com/issues/show_bug.cgi?id=11161



--- Comment #4 from Jonathan M Davis <jmdavisProg at gmx.com> 2013-10-03 10:55:48 PDT ---
> 'alias this' should be considered for operator overloading. If not, sub-typing
by using 'alias this' won't work anymore.

I would expect sub-typing via alias this to only come into effect when the
values being compared were of different types. So,

struct S
{
    bool opEquals(ref const S) { return false; } 
}
struct T
{
    S s;
    int value;
    alias value this;
}

void main()
{
    S s1;
    S s2;
    int i;

    //Uses built-in opEquals, because the types match
    auto result1 = s1 == s2;

    //Uses alias this because it's not comparing with another S
    auto result2 = s1 == i;
}

I don't understand why alias this would even come into effect until you're
dealing with a type other than S. As I understand it, alias this is for when
the original type doesn't work, in which case the conversion is done if that
will work instead. And that doesn't apply here.

I'd also expect the compiler to always define a default opEquals for structs if
you don't define one. And that's what it does when there's no alias this
involved.

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