There is a line of C90 DRs and associated textual history (compare the relevant text in C90 and C99, or see my comparison of it in WG14 reflector message 11100 (18 Apr 2006)) to the effect of bit-fields acting like they have a type with the given number of bits; that line is what's followed by GCC for C. The choice of type for a bit-field (possibly separate from declared type) was left explicitly implementation-defined after DR#315; that is, if an implementation allows implementation-defined declared types as permitted by C99 and later, whether the actual type of the bit-field in question is the declared type or has the specified number of bits is also implementation-defined. The point in DR#120 regarding assignment to bit-fields still applies in C11: nothing other than the semantics of conversion to a type with the given number of bits defines how the value to be stored in a bit-field is computed if the stored value is not in range.
C++ chose a different route from those C90 DRs, of the width explicitly not being part of the type of the bit-field. I don't know what if anything in C++ explicitly resolves the C90 DR#120 issue and defines the results of storing not-exactly-representable values in a bit-field. -- Joseph S. Myers jos...@codesourcery.com
