Re: isinf, isnan, signbit: Make more C++ safe

[Bruno Haible via Gnulib discussion list]

> 2026-02-03  Bruno Haible  <br...@clisp.org>
> 
>       isnan: Make more C++ safe.
>       * lib/math.in.h (isnan): In C++ mode, define as a template with three
>       instantiations, instead of as a macro.

This patch produced a compilation error in C++ mode on CentOS 7 and NetBSD 10:

  In file included from ../gllib/math.h:46,
                   from ../../gltests/test-math-h-c++.cc:22:
  ../gllib/math.h:3213:27: error: expected unqualified-id before ‘sizeof’
   3213 | template <typename T> int isnan (T);
        |                           ^~~~~
  ../gllib/math.h:3213:27: error: expected ‘)’ before ‘sizeof’

Fixed as follows.


2026-02-09  Bruno Haible  <br...@clisp.org>

        isnan: Fix compilation error in C++ mode on NetBSD (regr. 2026-02-03).
        * lib/math.in.h (isnan): Undefine before defining as a template.

diff --git a/lib/math.in.h b/lib/math.in.h
index 3b33ab4a9e..4174445d38 100644
--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -2650,6 +2650,7 @@ _GL_EXTERN_C int rpl_isnanl (long double x) 
_GL_ATTRIBUTE_CONST;
 #   define gl_isnan_l(x) rpl_isnanl (x)
 #  endif
 #  ifdef __cplusplus
+#   undef isnan
 template <typename T> int isnan (T);
 template <> inline int isnan<float> (float x) { return gl_isnan_f (x); }
 template <> inline int isnan<double> (double x) { return gl_isnan_d (x); }
@@ -2663,6 +2664,7 @@ template <> inline int isnan<long double> (long double x) 
{ return gl_isnan_l (x
 #  endif
 # elif (__GNUC__ >= 4) || (__clang_major__ >= 4)
 #  ifdef __cplusplus
+#   undef isnan
 template <typename T> int isnan (T);
 template <> inline int isnan<float> (float x) { return __builtin_isnan (x); }
 template <> inline int isnan<double> (double x) { return __builtin_isnan (x); }