[Cfp-interest] C1x's _Generic and totalorder()

[Joel C. Salomon]

On 07/14/2011 05:08 PM, Stephen Canon amended my suggestion thus:
> #define totalorder(x,y) \
>     _Generic((x), float: _Generic((y), \
>                                   float: totalorderf, \
>                                   double: _Static_assert(0, "type mismatch")), \
>                   double: _Generic((y), \
>                                    float: _Static_assert(0, "type mismatch")), \
>                                    double: totalorder))((x),(y))
>     
> but it still fails for exactly the reason that Joel highlighted;
> _Generics expand to expressions, which _Static_assert is not.

On comp.lang.c, Jens Gustedt suggested using an invalid constant
expression, e.g., sizeof (double[-1]), in place of the _Static_assert.
Ben Bacarisse was unsure whether that would work (see the thread "Static
Assertions that are Expressions",
<http://groups.google.com/group/comp.lang.c/browse_thread/thread/0e0f381ab1060113>,
for details), and suggested instead:

> Why not just remove the option of the "bad"  types in the inner
> _Generic expressions?  A type for y that is not compatible with the
> only listed option is a constraint violation so a diagnostic is
> required. 

I don't have a compiler that implements _Generic, so I can't test this:

#define totalorder(x,y) _Generic((x), \
	float:  _Generic((y), float: totalorderf), \
	double: _Generic((y), double: totalorder)) ((x),(y))

I'm particularly unsure whether this will catch the incorrect usage
	totalorder(1.0, 2.0f)
or whether the float argument will be considered "compatible" with the
double generic-association.

--Joel Salomon