This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

587. Lvalue operands of a conditional expression differing only in cv-qualification

[Voted into WP at October, 2009 meeting.]

Consider the following example:

    template <typename T>
    const T* f(bool b) {
        static T t1 = T();
        static const T t2 = T();
        return &(b ? t1 : t2);  // error?
    }

According to 7.6.16 [expr.cond], this function is well-formed if T is a class type and ill-formed otherwise. If the second and third operands of a conditional expression are lvalues of the same class type except for cv-qualification, the result of the conditional expression is an lvalue; if they are lvalues of the same non-class type except for cv-qualification, the result is an rvalue.

This difference seems gratuitous and should be removed.

Proposed resolution (June, 2009):

Change 7.6.16 [expr.cond] paragraph 3 as follows:

Otherwise, if the second and third operand have different types, and either has (possibly cv-qualified) class type, or if both are lvalues of the same type except for cv-qualification, an attempt is made to convert each of those operands to the type of the other. The process for determining whether an operand expression E1 of type T1 can be converted to match an operand expression E2 of type T2 is defined as follows:

• If E2 is an lvalue: E1 can be converted to match E2 if E1 can be implicitly converted (7.3 [conv]) to the type “lvalue reference to T2”, subject to the constraint that in the conversion the reference must bind directly (9.4.4 [dcl.init.ref]) to E1.

• If E2 is an rvalue, or if the conversion above cannot be done and at least one of the operands has (possibly cv-qualified) class type:

• ...