CWG Issue 2484 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-05

2484.char8_t and char16_t in integral promotions

Section: 7.3.7 [conv.prom]Status: CD6Submitter: Richard SmithDate: 2021-04-01

[Accepted as a DR at the October, 2021 meeting.]

According to 7.3.7 [conv.prom] paragraphs 1-2,

A prvalue of an integer type other than bool,char16_t, char32_t, or wchar_twhose integer conversion rank (6.9.6 [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.

A prvalue of type char16_t, char32_t, or wchar_t (6.9.2 [basic.fundamental]) 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 char16_t, char32_t, orwchar_t can be converted to a prvalue of its underlying type.

Because of its omission from the list of excluded types (perhaps as an oversight when it was added), char8_t is handled in the first paragraph. However, char16_t falls into the second paragraph, even though it is guaranteed to be convertible to int or unsigned int. This seems inconsistent, so perhaps char8_t should be moved to the second paragraph or char16_t moved to the first?

Notes from the August, 2021 teleconference:

char8_t should be handled by the second paragraph by including it in all three lists of types in the two paragraphs.

Proposed resolution (August, 2021):

Change 7.3.7 [conv.prom] paragraphs 1 and 2 as follows:

A prvalue of an integer type other than bool, char8_t, char16_t, char32_t, or wchar_twhose integer conversion rank (6.9.6 [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.

A prvalue of type char8_t, char16_t, char32_t, or wchar_t (6.9.2 [basic.fundamental]) 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 its underlying type.