Test return type 'int' only for glibc + cxx03

libcxx/test/std/numerics/c.math/cmath.pass.cpp

@@ -705,15 +705,16 @@ void test_isinf()
     static_assert((std::is_same<decltype(std::isinf((float)0)), bool>::value), "");
 
     typedef decltype(std::isinf((double)0)) DoubleRetType;
-#if !defined(__linux__) || defined(__clang__)
-    static_assert((std::is_same<DoubleRetType, bool>::value), "");
-#else
+#if defined(__GLIBC__) && defined(_LIBCPP_CXX03_LANG) && !defined(_LIBCPP_PREFERRED_OVERLOAD)
     // GLIBC < 2.23 defines 'isinf(double)' with a return type of 'int' in
     // all C++ dialects. The test should tolerate this when libc++ can't work
-    // around it.
+    // around it via `_LIBCPP_PREFERRED_OVERLOAD`, which is only available
+    // in modern versions of Clang, and not elsewhere.
     // See: https://sourceware.org/bugzilla/show_bug.cgi?id=19439
     static_assert((std::is_same<DoubleRetType, bool>::value
                 || std::is_same<DoubleRetType, int>::value), "");
+#else
+    static_assert((std::is_same<DoubleRetType, bool>::value), "");
 #endif
 
     static_assert((std::is_same<decltype(std::isinf(0)), bool>::value), "");
@@ -791,15 +792,16 @@ void test_isnan()
     static_assert((std::is_same<decltype(std::isnan((float)0)), bool>::value), "");
 
     typedef decltype(std::isnan((double)0)) DoubleRetType;
-#if !defined(__linux__) || defined(__clang__)
-    static_assert((std::is_same<DoubleRetType, bool>::value), "");
-#else
-    // GLIBC < 2.23 defines 'isinf(double)' with a return type of 'int' in
+#if defined(__GLIBC__) && defined(_LIBCPP_CXX03_LANG) && !defined(_LIBCPP_PREFERRED_OVERLOAD)
+    // GLIBC < 2.23 defines 'isnan(double)' with a return type of 'int' in
     // all C++ dialects. The test should tolerate this when libc++ can't work
-    // around it.
+    // around it via `_LIBCPP_PREFERRED_OVERLOAD`, which is only available
+    // in modern versions of Clang, and not elsewhere.
     // See: https://sourceware.org/bugzilla/show_bug.cgi?id=19439
     static_assert((std::is_same<DoubleRetType, bool>::value
                 || std::is_same<DoubleRetType, int>::value), "");
+#else
+    static_assert((std::is_same<DoubleRetType, bool>::value), "");
 #endif
 
     static_assert((std::is_same<decltype(std::isnan(0)), bool>::value), "");