Re: [PATCH next 11/14] bit: Strengthen compile-time tests in GENMASK() and BIT()

[David Laight <david.laight.linux@gmail.com>]

On Thu, 22 Jan 2026 16:26:44 +0200
Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> On Thu, Jan 22, 2026 at 10:33:37AM +0000, David Laight wrote:
> > On Thu, 22 Jan 2026 12:41:22 +0800
> > kernel test robot <lkp@intel.com> wrote:  
> 
> > > sparse warnings: (new ones prefixed by >>)
> > >    kernel/kthread.c: note: in included file (through include/linux/bitmap.h, include/linux/cpumask.h, arch/mips/include/asm/processor.h, ...):  
> > > >> include/linux/find.h:209:45: sparse: sparse: Variable length array is used.    
> > 
> > Can we stop sparse complaining about sizeof(VLA) ?  
> 
> First of all, the LKP should install the fork of sparse by Al Viro. That will
> fix tons of warnings that are related to modules and speed up the process
> itself.
> 

The problem I have is I want to generate a compile time error inside:
	__builtin_choose_expr(__is_constexpr(x), x && error_a(),
		statically_true(y) && error_b());
Neither static_assert() nor a negative bitfield can be used in error_a()
because they are errors when x isn't isn't a 'constexpr',
There is less of a problem for error_b() it can contain a function call,
but can't contain {( ...)}.

One of the compiler complained about sizeof (char [-!!(expr)]) as well.
I'm not sure of anything else that can be use to get an error.
I could use 0 >> -1 but that is only a warning, at least the error message
is related to the bug.

Any other ideas?

	David