diff --git a/lib/math.in.h b/lib/math.in.h
--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -2396,13 +2396,6 @@
     sizeof (x) == sizeof (double) ? gl_signbitd (x) : \
     gl_signbitf (x))
 # endif
-# ifdef __cplusplus
-#  if defined signbit || defined GNULIB_NAMESPACE
-_GL_MATH_CXX_REAL_FLOATING_DECL_1 (signbit)
-#   undef signbit
-_GL_MATH_CXX_REAL_FLOATING_DECL_2 (signbit)
-#  endif
-# endif
 #elif defined GNULIB_POSIXCHECK
 # if defined signbit
 _GL_WARN_REAL_FLOATING_DECL (signbit);