This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

976. Deduction for const T& conversion operators

[Voted into WP at March, 2010 meeting.]

Consider this program:

    struct F {
       template<class T>
       operator const T&() { static T t; return t; }
    };

    int main() {
       F f;
       int i = f;   // ill-formed
    }

It's ill-formed, because according to 13.10.3.4 [temp.deduct.conv], we try to match const T with int.

(The reference got removed from P because of paragraph 3, but the const isn't removed, because bullet 2.3 comes before paragraph 3 and thus isn't applied any more.)

Changing the declaration of the conversion operator to

   operator T&() { ... }

makes the program compile, which is counter-intuitive to me: I'm in an rvalue (read-only) context, and I can use a conversion to T&, but I can't use a conversion to const T&?

Proposed resolution (February, 2010):

Change 13.10.3.4 [temp.deduct.conv] paragraphs 1-3 as follows, inserting a new paragraph between the current paragraphs 1 and 2:

Template argument deduction is done by comparing the return type of the conversion function template (call it P) with the type that is required as the result of the conversion (call it A) as described in 13.10.3.6 [temp.deduct.type].

If A is not a reference type:

• If P is an array type, the pointer type produced by the array-to-pointer standard conversion (7.3.3 [conv.array]) is used in place of P for type deduction; otherwise,

• If P is a function type, the pointer type produced by the function-to-pointer standard conversion (7.3.4 [conv.func]) is used in place of P for type deduction; otherwise,

• If P is a cv-qualified type, the top level cv-qualifiers of P's type are ignored for type deduction.

If A is a cv-qualified type, the top level cv-qualifiers of A's type are ignored for type deduction. If A is a reference type, the type referred to by A is used for type deduction. If P is a reference type, the type referred to by P is used for type deduction.

(This resolution also resolves issues 493 and 913.)

[Drafting note: This change intentionally reverses the resolution of issue 322 (and applies it in a different form).]