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
[Accepted as a DR at the June, 2018 (Rapperswil) meeting.]
The following example is ill-formed:
struct A {};
struct B : A {};
using T = const B;
A a = true ? A() : T();
We don't convert from A to T because we can't form an implicit conversion sequence. We don't convert from T to A because T is more cv-qualified (even though we could form an implicit conversion sequence). It would seem reasonable to accept this case; it seems that we should only be using cv-qualifiers as a tie-breaker if the class types are otherwise the same.
Proposed resolution (March, 2018):
Change 7.6.16 [expr.cond] bullet 4.3 as follows:
...Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:
...
If E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:
if T1 and T2 are the same class type (ignoring cv-qualification)
, or one is a base class of the other,and T2 is at least as cv-qualified as T1 , the target type is T2,otherwise, if T2 is a base class of T1, the target type is cv1 T2, where cv1 denotes the cv-qualifiers of T1,
otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions.