[Voted into WP at October 2003 meeting.]

Proposed resolution (April 2003):

Add to the end of 13.10.3.4 [temp.deduct.conv] (as a new paragraph following paragraph 3):

When the deduction process requires a qualification conversion for a pointer or pointer to member type as described above, the following process is used to determine the deduced template argument values:

If A is a type cv_1,0 pointer to ... cv _1,n-1 pointer to cv_1,n T1

and P is a type cv_2,0 pointer to ... cv_2,n-1 pointer to cv_2,n T2

The cv-unqualified T1 and T2 are used as the types of A and P respectively for type deduction.

[Example:

struct A {
	template <class T> operator T***();
};

A a;
const int * const * const * p1 = a;  // T is deduced as int, not const int

-- end example]

Notes from October 2002 meeting:

There was consensus on having the deduced type be "int" in the above.

We ran into an issue concerning qualification conversions when doing template argument deduction for conversion functions.

The question is: What is the type of T in the conversion functions called by this example? Is T "int" or "const int"?

If T is "int", the conversion function in class A works and the one in class B fails (because the return expression cannot be converted to the return type of the function). If T is "const int", A fails and B works.

Because the qualification conversion is performed on the result of the conversion function, I see no benefit in deducing T as const int.

In addition, I think the code in class A is more likely to occur than the code in class B. If the author of the class was planning on returning a pointer to a const entity, I would expect the function to have been written with a const in the return type.

Consequently, I believe the correct result should be that T is int.

struct A {
	template <class T> operator T***() {
		int*** p = 0;
		return p;
	}
};

struct B {
	template <class T> operator T***() {
		const int*** p = 0;
		return p;
	}
};

int main()
{
	A a;
	const int * const * const * p1 = a;
	B b;
	const int * const * const * p2 = b;
}

We have just implemented this feature, and pending clarification by the committee, we deduce T as int. It appears that g++ and the Sun compiler deduce T as const int.

One way or the other, I think the standard should be clarified to specify how cases like this should be handled.