Do not use const attribute for nan functions (bug 23277).

As in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86113 for
__builtin_nan, bits/mathcalls.h wrongly declares the nan function with
the __const__ attribute.  Because the function reads memory pointed to
by an argument, it's only pure, not const.  This patch removes the
incorrect attribute and adds a testcase for the bug.  No __pure__
attribute is added to replace the incorrect __const__ one, since that
would introduce problems when using GCC versions that have the
incorrect built-in __const__ attribute and warn for the combination of
those two attributes.

Tested for x86_64.

	[BZ #23277]
	* math/bits/mathcalls.h [__USE_ISOC99] (nan): Do not use __const__
	attribute.
	* math/test-nan-const.c: New file.
	* math/Makefile (tests): Add test-nan-const.
	(CFLAGS-test-nan-const.c): New variable.

1 files changed, 1 insertions, 1 deletions

diff --git a/math/bits/mathcalls.h b/math/bits/mathcalls.h
index 92003639ab..cf87313e2f 100644
--- a/math/bits/mathcalls.h
+++ b/math/bits/mathcalls.h
@@ -198,7 +198,7 @@ __MATHCALLX (copysign,, (_Mdouble_ __x, _Mdouble_ __y), (__const__));
 
 #ifdef __USE_ISOC99
 /* Return representation of qNaN for double type.  */
-__MATHCALLX (nan,, (const char *__tagb), (__const__));
+__MATHCALL (nan,, (const char *__tagb));
 #endif