[Bug libc/15720] New: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__

["bugdal at aerifal dot cx" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15720

            Bug ID: 15720
           Summary: stdc-predef.h wrongly defines __STDC_IEC_559_COMPLEX__
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: bugdal at aerifal dot cx
                CC: drepper.fsp at gmail dot com

Based on the comment in stdc-predef.h, somebody apparently assumed
__STDC_IEC_559_COMPLEX__ means the implementation supports complex arithmetic.
This is not what it means. Complex arithmetic is mandatory in C99, and in C11
implementations that lack complex arithmetic must define __STDC_NO_COMPLEX__.
The meaning of __STDC_IEC_559_COMPLEX__ is that the implementation conforms to
Annex G, which among other things, requires the existence of the
otherwise-optional _Imaginary types which GCC does not support.

This is a fairly serious issue for applications, since on an implementation
conforming to Annex G, I*INFINITY is interpreted as intended, whereas on an
implementation lacking _Imaginary types and defining I as _Complex_I,
I*INFINITY results in a NAN due to the 0*INFINITY term.

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