math: Fix errors in C++ mode on glibc systems with clang

[Bruno Haible]

Building a complete gnulib testdir on a glibc system with clang 13, I see
these errors:

../../gltests/test-math-c++.cc:388:22: error: no member named 'isfinite' in 
namespace 'gnulib'
../../gltests/test-math-c++.cc:395:22: error: no member named 'isinf' in 
namespace 'gnulib'
../../gltests/test-math-c++.cc:402:22: error: no member named 'isnan' in 
namespace 'gnulib'
../../gltests/test-math-c++.cc:409:22: error: no member named 'signbit' in 
namespace 'gnulib'

This patch fixes them. I don't think it's possible to really define
the gnulib namespace member as desired, in this situation.


2022-04-13  Bruno Haible  <bruno@clisp.org>

        math: Fix errors in C++ mode on glibc systems with clang.
        * lib/math.in.h (GNULIB_NAMESPACE_LACKS_ISFINITE,
        GNULIB_NAMESPACE_LACKS_ISINF, GNULIB_NAMESPACE_LACKS_ISNAN,
        GNULIB_NAMESPACE_LACKS_SIGNBIT): New macros.
        * tests/test-math-c++.cc (isfinite): Skip test if
        GNULIB_NAMESPACE_LACKS_ISFINITE is 1.
        (isinf): Skip test if GNULIB_NAMESPACE_LACKS_ISINF is 1.
        (isnan): Skip test if GNULIB_NAMESPACE_LACKS_ISNAN is 1.
        (signbit): Skip test if GNULIB_NAMESPACE_LACKS_SIGNBIT is 1.

diff --git a/lib/math.in.h b/lib/math.in.h
index ccc3584802..a74a95da3e 100644
--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -2433,6 +2433,7 @@ _GL_MATH_CXX_REAL_FLOATING_DECL_1 (isfinite)
      functions.  */
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isfinite, rpl_isfinite, bool)
 #    define isfinite rpl_isfinite
+#    define GNULIB_NAMESPACE_LACKS_ISFINITE 1
 #   else
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isfinite, isfinite, bool)
 #   endif
@@ -2467,6 +2468,7 @@ _GL_MATH_CXX_REAL_FLOATING_DECL_1 (isinf)
      functions.  */
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isinf, rpl_isinf, bool)
 #    define isinf rpl_isinf
+#    define GNULIB_NAMESPACE_LACKS_ISINF 1
 #   else
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isinf, isinf, bool)
 #   endif
@@ -2594,6 +2596,7 @@ _GL_MATH_CXX_REAL_FLOATING_DECL_1 (isnan)
      functions.  */
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan, rpl_isnan, bool)
 #    define isnan rpl_isnan
+#    define GNULIB_NAMESPACE_LACKS_ISNAN 1
 #   else
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (isnan, isnan, bool)
 #   endif
@@ -2677,6 +2680,7 @@ _GL_MATH_CXX_REAL_FLOATING_DECL_1 (signbit)
      functions.  */
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (signbit, rpl_signbit, bool)
 #    define signbit rpl_signbit
+#    define GNULIB_NAMESPACE_LACKS_SIGNBIT 1
 #   else
 _GL_MATH_CXX_REAL_FLOATING_DECL_2 (signbit, signbit, bool)
 #   endif
diff --git a/tests/test-math-c++.cc b/tests/test-math-c++.cc
index e9a7bfa6a5..6749fa58a1 100644
--- a/tests/test-math-c++.cc
+++ b/tests/test-math-c++.cc
@@ -385,28 +385,36 @@ SIGNATURE_CHECK (GNULIB_NAMESPACE::truncl, long double, 
(long double));
 # ifdef isfinite
 #  warning "isfinite should not be a macro in C++"
 # endif
+# if !GNULIB_NAMESPACE_LACKS_ISFINITE
 REAL_FLOATING_CHECK (isfinite, bool, (float), bool, (double), bool, (long 
double));
+# endif
 #endif
 
 #if GNULIB_TEST_ISINF
 # ifdef isinf
 #  warning "isinf should not be a macro in C++"
 # endif
+# if !GNULIB_NAMESPACE_LACKS_ISINF
 REAL_FLOATING_CHECK (isinf, bool, (float), bool, (double), bool, (long 
double));
+# endif
 #endif
 
 #if GNULIB_TEST_ISNAN
 # ifdef isnan
 #  warning "isnan should not be a macro in C++"
 # endif
+# if !GNULIB_NAMESPACE_LACKS_ISNAN
 REAL_FLOATING_CHECK (isnan, bool, (float), bool, (double), bool, (long 
double));
+# endif
 #endif
 
 #if GNULIB_TEST_SIGNBIT
 # ifdef signbit
 #  warning "signbit should not be a macro in C++"
 # endif
+# if !GNULIB_NAMESPACE_LACKS_SIGNBIT
 REAL_FLOATING_CHECK (signbit, bool, (float), bool, (double), bool, (long 
double));
+# endif
 #endif