On 29.7.2013 14:27, Raphael Kubo da Costa wrote:
David Chisnall <thera...@freebsd.org> writes:
On 28 Jul 2013, at 22:27, Raphael Kubo da Costa <rak...@freebsd.org> wrote:
This seems to have been committed in r253321, and broke some code that
was working with r253320; namely, some code in x11/kde4-workspace
includes math.h and calls isnan() with a const double.
Please provide a test case. Specifically, I need to know what
language dialect this is using, because I have tested including math.h
and calling isnan(double) with c89, gnu89, c99, c11, c++03 and c++11
on gcc (for the modes that it supports) and clang.
I get the following results with and without -std=c++11 and/or
-stdlib=libc++:
% cat isnan.cc
#include <math.h>
int main() {
const double d = 42.0;
return isnan(d) ? 1 : 0;
}
% clang++ isnan.cc
isnan.cc:4:10: error: controlling expression type 'const double' not
compatible with any generic association type
return isnan(d) ? 1 : 0;
^~~~~~~~
/usr/include/math.h:109:2: note: expanded from macro 'isnan'
__fp_type_select(x, __inline_isnanf, __inline_isnan,
__inline_isnanl)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/math.h:86:49: note: expanded from macro '__fp_type_select'
#define __fp_type_select(x, f, d, ld) _Generic((0,(x)),
\
^~~~~
1 error generated.
In the first place C++ code should be using cmath instead of math.h.
That said, this is a result of just an another wonderful little
difference between C and C++. C standard explicitly states that comma
operator does not yield an lvalue but in C++ it can if the right most
operand is an lvalue. If C++ compatibility is desired then unary +
operator (e.g. _Generic(+(x),...) could be used for the same effect,
only downside with this is that the integer arguments are promoted, but
that doesn't matter in this case.
Pasi
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