math: Define iszero as a function template for C++ [BZ #20715]

This increases compatibility with C++ code which is forced to
compile with _GNU_SOURCE.

2016-10-21  Florian Weimer  <fweimer@redhat.com>

	[BZ #20715]
	* math/math.h [__cplusplus] (iszero): Define as function template.
	* math/Makefile [CXX] (tests): Add test-math-iszero.
	(CFLAGS-test-math-iszero.cc): Set.
	* math/test-math-iszero.cc: New file.

diff --git a/math/Makefile b/math/Makefile
index 2aa6a34..7cecba5 100644
--- a/math/Makefile
+++ b/math/Makefile
@@ -165,7 +165,7 @@ tests-static = test-fpucw-static test-fpucw-ieee-static \
 	       test-signgam-ullong-static test-signgam-ullong-init-static
 
 ifneq (,$(CXX))
-tests += test-math-isinff
+tests += test-math-isinff test-math-iszero
 endif
 
 ifneq (no,$(PERL))
@@ -267,6 +267,7 @@ CFLAGS-test-signgam-ullong-static.c = -std=c99
 CFLAGS-test-signgam-ullong-init-static.c = -std=c99
 
 CFLAGS-test-math-isinff.cc = -std=gnu++11
+CFLAGS-test-math-iszero.cc = -std=gnu++11
 
 CFLAGS-test-iszero-excess-precision.c = -fexcess-precision=standard
 CFLAGS-test-iseqsig-excess-precision.c = -fexcess-precision=standard
diff --git a/math/math.h b/math/math.h
index 880b4a0..0220d08 100644
--- a/math/math.h
+++ b/math/math.h
@@ -335,11 +335,25 @@ enum
 # define issubnormal(x) (fpclassify (x) == FP_SUBNORMAL)
 
 /* Return nonzero value if X is zero.  */
-# ifdef __SUPPORT_SNAN__
-#  define iszero(x) (fpclassify (x) == FP_ZERO)
-# else
-#  define iszero(x) (((__typeof (x)) (x)) == 0)
-# endif
+# ifndef __cplusplus
+#  ifdef __SUPPORT_SNAN__
+#   define iszero(x) (fpclassify (x) == FP_ZERO)
+#  else
+#   define iszero(x) (((__typeof (x)) (x)) == 0)
+#  endif
+# else	/* __cplusplus */
+__END_DECLS
+template <class __T> inline bool
+iszero (__T __val)
+{
+#  ifdef __SUPPORT_SNAN__
+  return fpclassify (__val) == FP_ZERO;
+#  else
+  return __val == 0;
+#  endif
+}
+__BEGIN_DECLS
+# endif	/* __cplusplus */
 #endif /* Use IEC_60559_BFP_EXT.  */
 
 #ifdef	__USE_MISC
diff --git a/math/test-math-iszero.cc b/math/test-math-iszero.cc
new file mode 100644
index 0000000..5de251a
--- /dev/null
+++ b/math/test-math-iszero.cc
@@ -0,0 +1,87 @@
+/* Test for the C++ implementation of iszero.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#define _GNU_SOURCE 1
+#include <math.h>
+#include <stdio.h>
+
+#include <limits>
+
+static bool errors;
+
+static void
+check (int actual, int expected, const char *actual_expr, int line)
+{
+  if (actual != expected)
+    {
+      errors = true;
+      printf ("%s:%d: error: %s\n", __FILE__, line, actual_expr);
+      printf ("%s:%d:   expected: %d\n", __FILE__, line, expected);
+      printf ("%s:%d:   actual: %d\n", __FILE__, line, actual);
+    }
+}
+
+#define CHECK(actual, expected) \
+  check ((actual), (expected), #actual, __LINE__)
+
+template <class T>
+static void
+check_type ()
+{
+  typedef std::numeric_limits<T> limits;
+  CHECK (iszero (T{}), 1);
+  CHECK (iszero (T{0}), 1);
+  CHECK (iszero (T{-0.0}), 1);
+  CHECK (iszero (T{1}), 0);
+  CHECK (iszero (T{-1}), 0);
+  CHECK (iszero (limits::min ()), 0);
+  CHECK (iszero (-limits::min ()), 0);
+  CHECK (iszero (limits::max ()), 0);
+  CHECK (iszero (-limits::max ()), 0);
+  if (limits::has_infinity)
+    {
+      CHECK (iszero (limits::infinity ()), 0);
+      CHECK (iszero (-limits::infinity ()), 0);
+    }
+  CHECK (iszero (limits::epsilon ()), 0);
+  CHECK (iszero (-limits::epsilon ()), 0);
+  if (limits::has_quiet_NaN)
+    CHECK (iszero (limits::quiet_NaN ()), 0);
+  if (limits::has_signaling_NaN)
+    CHECK (iszero (limits::signaling_NaN ()), 0);
+  if (limits::has_signaling_NaN)
+    CHECK (iszero (limits::signaling_NaN ()), 0);
+  CHECK (iszero (limits::denorm_min ()),
+         std::numeric_limits<T>::has_denorm == std::denorm_absent);
+  CHECK (iszero (-limits::denorm_min ()),
+         std::numeric_limits<T>::has_denorm == std::denorm_absent);
+}
+
+static int
+do_test (void)
+{
+  check_type<float> ();
+  check_type<double> ();
+#ifndef NO_LONG_DOUBLE
+  check_type<long double> ();
+#endif
+  return errors;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
