CPP11 — don't understand the last paragraph of 1.18.2:

"Both interfaces that are derived from one or more abstract interfaces,
and valuetypes that support one or more abstract interfaces support
implicit widening to the _ptr type for each abstract interface base
class. Specifically, the T* for valuetype T and the T_ptr type for
interface type T support implicit widening to the Base_ptr type for
abstract interface type Base. The only exception to this rule is for
valuetypes that directly or indirectly support one or more regular
interface types (see Section 1.17.9, Valuetype Inheritance, on page
1-83). In these cases, it is the object reference for the valuetype, not
the pointer to the valuetype, that supports widening to the abstract
interface base."

This seems to prohibit widening from a valuetype pointer to an abstract
interface _ptr if the valuetype happens to support a normal interface.
I don't understand the restriction. This seems to prohibit the
programmer making a choice of using value or reference semantics in the
case of diamond inheritence of an abstract interface:

// IDL
abstract interface A {
};

interface I : A {
};

valuetype V1 : supports A {
};

valuetype V2 : supports I {
};

This means that I must always pass V2 valuetypes by reference to
operations expecting an A, and cannot choose to pass V2 valuetypes by
value instead. Why was this restriction added and what would be the
problem with relaxing it in this case?

see below