Structs as Keys for AAs

[Q. Schroll via Digitalmars-d]

In [1] it says at 5. that

> For this reason, and for legacy reasons, an associative array 
> key is not allowed to define a specialized opCmp, but omit a 
> specialized opEquals. This restriction may be removed in future 
> versions of D.

I'm not completely sure what that means. Does "specialized" mean 
"user-defined"? I just challenged the spec and found an error by 
the way: [2]. Apart from that, it compiles.

For 5. I used

struct Key
{
     int id;
     string tag;

     int opCmp(const Key other) const
     {
         return this.id < other.id ? -1 : this.id == other.id ?  0 
: 1;
     }

     bool opEquals(ref const Key other) const @safe pure nothrow
     {
         return this.id == other.id;
     }

     size_t toHash() const @safe pure nothrow
     {
         return id;
     }
}

as a key type. To me the part "is not allowed to define a 
specialized opCmp" is clearly wrong, either a compiler bug or an 
error in the spec.

Concerning opEquals and opCmp in general: Why isn't opEquals 
lowered to opCmp returning 0 if not present?

[1] https://dlang.org/spec/hash-map.html#using_struct_as_key
[2] https://github.com/dlang/dlang.org/pull/1861