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Date: Tue, 20 Oct 2009 17:41:48 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 00/25] VMState cleanups and conversion of
	network drivers
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To: Jamie Lokier <jamie@shareable.org>
Cc: qemu-devel@nongnu.org, Juan Quintela <quintela@redhat.com>

On 10/19/2009 09:15 PM, Jamie Lokier wrote:
> __builtin_types_compatible_p(), __builtin_choose_exper and
> __attribute__((__error__)) are good for informative error messages.

Unfortunately __attribute__((__error__)) cannot be applied in this case 
for several reasons:

1) it cannot apply to types so you cannot do

extern void compile_error (void);

#define raise_compile_error(x) \
   (((void __attribute__ ((error(x))) (*) (void)) x) ())

2) it only operates for calls, so you cannot use it in initializers

I whipped up a patch to add to GCC __builtin_compile_error, to be used 
inside __builtin_choose_expr:

#define typecheck(t1, t2) \
   __builtin_choose_expr (__builtin_types_compatible(t1, t2), (void)0, \
                          __builtin_compile_error (#t1 " and " #t2     \
                              "are not compatible types"))

#define if_types_compatible(t1, t2, value) (typecheck (t1, t2), value)

and I will send it upstream, but it will be a long time before it is in 
a released version (especially since GCC is currently in bug-fixing-only 
mode)---anyway Jamie got the array case to work, AFAIU.

Paolo