Cannot dup an associative array but why?

[Jean-Louis Leroy via Digitalmars-d-learn]

Is there something special about ClassInfo that confuses? Look at 
this example:

struct Foo
{
}

class Bar
{
}

void main()
{
  Foo*[Bar] a;
  auto aa = a.dup; // OK
  Foo*[ClassInfo] b; // Error: static assert  "cannot call 
Foo*[TypeInfo_Class].dup because Foo* is not copyable"
  auto bb = b.dup; // instantiated from here: 
dup!(Foo*[TypeInfo_Class], TypeInfo_Class, Foo*)}
}

'Foo*' is not copyable? Except that it is if the key is something 
else than ClassInfo? ClassInfo and Bar are both reference types 
though...