diff --git a/clang/lib/Headers/__clang_cuda_runtime_wrapper.h b/clang/lib/Headers/__clang_cuda_runtime_wrapper.h
--- a/clang/lib/Headers/__clang_cuda_runtime_wrapper.h
+++ b/clang/lib/Headers/__clang_cuda_runtime_wrapper.h
@@ -31,6 +31,11 @@
 // Include some forward declares that must come before cmath.
 #include <__clang_cuda_math_forward_declares.h>
 
+// Define __CUDACC__ early as libstdc++ standard headers with GNU extensions
+// enabled depend on it to avoid using __float128, which is unsupported in
+// CUDA.
+#define __CUDACC__
+
 // Include some standard headers to avoid CUDA headers including them
 // while some required macros (like __THROW) are in a weird state.
 #include <cmath>