diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index b57ff339990..72ec91949de 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -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 \