Bit-fields in integral promotions
7.3.7 [conv.prom]
Richard Smith

Created on 2021-04-01.00:00:00 last changed 3 weeks ago


Date: 2021-04-01.00:00:00

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.

Date User Action Args
2022-11-11 09:38:50adminsetstatus: open -> drafting
2021-04-01 00:00:00admincreate