--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,14 @@
 2010-01-11  Eric Blake  <ebb9@byu.net>
 
+	math: add portability warnings for classification macros
+	* modules/math (Depends-on): Add warn-on-use.
+	(Makefile.am): Provide new substitutions.
+	* m4/math_h.m4 (gl_MATH_H): Require inline.
+	* lib/math.in.h (_GL_WARN_REAL_FLOATING_DECL)
+	(_GL_WARN_REAL_FLOATING_IMPL): New helper macros.
+	(isfinite, isinf, isnan, signbit) [GNULIB_POSIXCHECK]: Use them to
+	implement warnings.
+
 	unistd: warn on use of environ without module
 	* modules/unistd (Depends-on): Add warn-on-use.
 	(Makefile.am): Provide new substitutions.

--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -32,6 +32,37 @@
 
 /* The definition of _GL_ARG_NONNULL is copied here.  */
 
+/* Helper macros to define a portability warning for the
+   classification macro FUNC called with VALUE.  POSIX declares the
+   classification macros with an argument of real-floating (that is,
+   one of float, double, or long double).  */
+#define _GL_WARN_REAL_FLOATING_DECL(func) \
+static inline int                                                   \
+rpl_ ## func ## f (float f)                                         \
+{                                                                   \
+  return func (f);                                                  \
+}                                                                   \
+static inline int                                                   \
+rpl_ ## func ## d (double d)                                        \
+{                                                                   \
+  return func (d);                                                  \
+}                                                                   \
+static inline int                                                   \
+rpl_ ## func ## l (long double l)                                   \
+{                                                                   \
+  return func (l);                                                  \
+}                                                                   \
+_GL_WARN_ON_USE (rpl_ ## func ## f, #func " is unportable - "       \
+                 "use gnulib module " #func " for portability");    \
+_GL_WARN_ON_USE (rpl_ ## func ## d, #func " is unportable - "       \
+                 "use gnulib module " #func " for portability");    \
+_GL_WARN_ON_USE (rpl_ ## func ## l, #func " is unportable - "       \
+                 "use gnulib module " #func " for portability")
+#define _GL_WARN_REAL_FLOATING_IMPL(func, value) \
+  (sizeof (value) == sizeof (float) ? rpl_ ## func ## f (value)     \
+   : sizeof (value) == sizeof (double) ? rpl_ ## func ## d (value)  \
+   : rpl_ ## func ## l (value))
+
 
 #ifdef __cplusplus
 extern "C" {
@@ -386,7 +417,11 @@
     gl_isfinitef (x))
 # endif
 #elif defined GNULIB_POSIXCHECK
-  /* How to override a macro?  */
+# if defined isfinite
+_GL_WARN_REAL_FLOATING_DECL (isfinite);
+#  undef isfinite
+#  define isfinite(x) _GL_WARN_REAL_FLOATING_IMPL (isfinite, x)
+# endif
 #endif
 
 
@@ -402,7 +437,11 @@
     gl_isinff (x))
 # endif
 #elif defined GNULIB_POSIXCHECK
-  /* How to override a macro?  */
+# if defined isinf
+_GL_WARN_REAL_FLOATING_DECL (isinf);
+#  undef isinf
+#  define isinf(x) _GL_WARN_REAL_FLOATING_IMPL (isinf, x)
+# endif
 #endif
 
 
@@ -501,7 +540,11 @@
     gl_isnan_f (x))
 # endif
 #elif defined GNULIB_POSIXCHECK
-  /* How to override a macro?  */
+# if defined isnan
+_GL_WARN_REAL_FLOATING_DECL (isnan);
+#  undef isnan
+#  define isnan(x) _GL_WARN_REAL_FLOATING_IMPL (isnan, x)
+# endif
 #endif
 
 
@@ -557,7 +600,11 @@
     gl_signbitf (x))
 # endif
 #elif defined GNULIB_POSIXCHECK
-  /* How to override a macro?  */
+# if defined signbit
+_GL_WARN_REAL_FLOATING_DECL (signbit);
+#  undef signbit
+#  define signbit(x) _GL_WARN_REAL_FLOATING_IMPL (signbit, x)
+# endif
 #endif
 
 

--- a/m4/math_h.m4
+++ b/m4/math_h.m4
@@ -1,4 +1,4 @@
-# math_h.m4 serial 14
+# math_h.m4 serial 15
 dnl Copyright (C) 2007-2010 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -8,6 +8,8 @@
 [
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
   gl_CHECK_NEXT_HEADERS([math.h])
+  AC_REQUIRE([AC_C_INLINE])
+
   AC_CACHE_CHECK([whether NAN macro works], [gl_cv_header_math_nan_works],
     [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[@%:@include <math.h>]],
       [[/* Solaris 10 has a broken definition of NAN.  Other platforms

--- a/modules/math
+++ b/modules/math
@@ -6,9 +6,10 @@
 m4/math_h.m4
 
 Depends-on:
+arg-nonnull
 include_next
 link-warning
-arg-nonnull
+warn-on-use
 
 configure.ac:
 gl_MATH_H
@@ -18,7 +19,7 @@
 
 # We need the following in order to create <math.h> when the system
 # doesn't have one that works with the given compiler.
-math.h: math.in.h $(LINK_WARNING_H) $(ARG_NONNULL_H)
+math.h: math.in.h $(LINK_WARNING_H) $(WARN_ON_USE_H) $(ARG_NONNULL_H)
 	$(AM_V_GEN)rm -f $@-t $@ && \
 	{ echo '/* DO NOT EDIT! GENERATED AUTOMATICALLY! */' && \
 	  sed -e 's|@''INCLUDE_NEXT_AS_FIRST_DIRECTIVE''@|$(INCLUDE_NEXT_AS_FIRST_DIRECTIVE)|g' \
@@ -81,6 +82,7 @@
 	      -e 's|@''REPLACE_TRUNCL''@|$(REPLACE_TRUNCL)|g' \
 	      -e '/definition of GL_LINK_WARNING/r $(LINK_WARNING_H)' \
 	      -e '/definition of _GL_ARG_NONNULL/r $(ARG_NONNULL_H)' \
+	      -e '/definition of _GL_WARN_ON_USE/r $(WARN_ON_USE_H)' \
 	      < $(srcdir)/math.in.h; \
 	} > $@-t && \
 	mv $@-t $@