This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-05-06
[Accepted as a DR at the June, 2023 meeting.]
According to 7.3.7 [conv.prom] paragraph 5,
A prvalue for an integral bit-field (11.4.10 [class.bit]) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.
This description has several problems. First, the “bit-field” semantic property only makes sense for glvalue expressions, so it's unclear why these rules are described as applying to a prvalue. Perhaps this should be rephrased as something like “An expression that was a bit-field glvalue prior to the application of the lvalue-to-rvalue conversion”?
Second, suppose that char32_t is wider than int. Per paragraph 2, a char32_t prvalue promotes to unsigned long (because unsigned long is necessarily at least 32 bits wide). But per paragraph 5, a char32_t : 32 bitfield does not promote. This seems inconsistent.
Finally, it is not clear that the usual integral promotions are not applied to bit-fields. This should be made explicit.
Proposed resolution (approved by CWG 2023-02-07):
Insert a paragraph before 7.3.7 [conv.prom] paragraph 1 as follows:
For the purposes of 7.3.7 [conv.prom], a converted bit-field is a prvalue that is the result of an lvalue-to-rvalue conversion (7.3.2 [conv.lval]) applied to a bit-field (11.4.10 [class.bit]).
Change in 7.3.7 [conv.prom] paragraph 1 as follows:
A prvalueofthat is not a converted bit-field and has an integer type other than bool, char8_t, char16_t, char32_t, or wchar_t whose integer conversion rank (6.8.6 [conv.rank]) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.
Change in 7.3.7 [conv.prom] paragraph 4 as follows:
A prvalue of an unscoped enumeration type whose underlying type is fixed (9.7.1 [dcl.enum]) can be converted to a prvalue of its underlying type. Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type. [ Note: A converted bit-field of enumeration type is treated as any other value of that type for promotion purposes. -- end note ]
Change in 7.3.7 [conv.prom] paragraph 5 as follows:
Aprvalue for an integral bit-field (11.4.10 [class.bit])converted bit-field of integral type can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field.If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has enumeration type, it is treated as any other value of that type for promotion purposes.
Move 7.3.7 [conv.prom] paragraph 2 after paragraph 5 and change as follows:
A prvalue of type char8_t, char16_t, char32_t, or wchar_t (6.8.2 [basic.fundamental]) (including a converted bit-field that was not already promoted to int or unsigned int according to the rules above) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long int, unsigned long int, long long int,orunsigned long long int. If none of the types in that list can represent all the values of its underlying type, a prvalue of type char8_t, char16_t, char32_t, or wchar_t can be converted to a prvalue of, or its underlying type.