On Wed, 24 Feb 2016, Wink Saville wrote: > > (c) nothing defines semantics of conversion of out-of-range values to > > bit-fields other than treating the width as part of the type (or in the > > case of _Bool bit-fields, having the special wording to make it explicit > > that those have the semantics of _Bool not the semantics of an ordinary > > unsigned integer type with the specified number of bits). > > > I don't see where in the standard it says the width of a bit field is > part of the type. > I see In Section 6.7.2.1 paragraphs 9, 10, 11, 12 requires the compiler to > know > the width, but I don't see where the width is part of the type. > > Could you point me to the appropriate paragraph(s)?
If you look at the London 2007 minutes <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1267.pdf> you'll see that at that time this was intended to be deliberately ambiguous, to support both C++ practice and the longstanding C intent as expressed in the responses to various C90 DRs (the difference, at that time, only mattering for implementation-defined bit-field types). The "interpreted as having a signed or unsigned integer type consisting of the specified number of bits" (paragraph 10), if applied for the purposes of interpreting other references to types in C11, has consequences including: (i) conversions to store a value in a bit-field follow the same rules as conversions to a normal integer type, (ii) a bit-field with width less than that of int also has integer conversion rank less than that of int, and so promotes to int under the integer promotions (even if the declared type is wider than int). (It also has the less intuitive consequences regarding bit-field types escaping from their bit-field context, as discussed in the present thread and elsewhere.) Without it being part of the type, making long:1 promote to int (as in C++) not long would require changes to the wording on promotions, and separate wording would be needed to make the results of assignments clear (and I'm not convinced that C++ has such wording in all cases where it is needed). -- Joseph S. Myers jos...@codesourcery.com
