@@ -425,9 +425,12 @@ _GLIBCXX_END_NAMESPACE_VERSION
// GLIBCXX_ABI Deprecated
// Define if compatibility should be provided for -mlong-double-64.
#undef _GLIBCXX_LONG_DOUBLE_COMPAT
+
// Define if compatibility should be provided for alternative 128-bit long
-// double formats.
+// double formats. Not possible for Clang until __ibm128 is supported.
+#ifndef __clang__
#undef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
+#endif
// Inline namespaces for long double 128 modes.
#if defined _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT \