diff --git a/libcxx/include/ccomplex b/libcxx/include/ccomplex
--- a/libcxx/include/ccomplex
+++ b/libcxx/include/ccomplex
@@ -19,6 +19,10 @@
 
 #include <complex>
 
+#if _LIBCPP_STD_VER > 17
+#error "C++20 removed the <ccomplex> header. Please, include <complex> or <complex.h> instead"
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/include/ciso646 b/libcxx/include/ciso646
--- a/libcxx/include/ciso646
+++ b/libcxx/include/ciso646
@@ -17,6 +17,10 @@
 
 #include <__config>
 
+#if _LIBCPP_STD_VER > 14
+#error "C++17 removed the <ciso646> header. If you are trying to detect the standard library, use <version> instead"
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstdbool b/libcxx/include/cstdbool
--- a/libcxx/include/cstdbool
+++ b/libcxx/include/cstdbool
@@ -21,6 +21,10 @@
 
 #include <__config>
 
+#if _LIBCPP_STD_VER > 17
+#error "C++20 removed the <cstdbool> header. bool, true and false are provided as compiler builtins"
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/include/ctgmath b/libcxx/include/ctgmath
--- a/libcxx/include/ctgmath
+++ b/libcxx/include/ctgmath
@@ -21,6 +21,10 @@
 #include <ccomplex>
 #include <cmath>
 
+#if _LIBCPP_STD_VER > 17
+#error "C++20 removed the <ctgmath> header. Please, use <tgmath.h> instead"
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif