diff --git a/libcxxabi/src/cxa_demangle.cpp b/libcxxabi/src/cxa_demangle.cpp
--- a/libcxxabi/src/cxa_demangle.cpp
+++ b/libcxxabi/src/cxa_demangle.cpp
@@ -10,6 +10,7 @@
 // file does not yet support:
 //   - C++ modules TS
 
+#include "demangle/DemangleConfig.h"
 #include "demangle/ItaniumDemangle.h"
 #include "__cxxabi_config.h"
 #include <cassert>
diff --git a/libcxxabi/src/demangle/DemangleConfig.h b/libcxxabi/src/demangle/DemangleConfig.h
--- a/libcxxabi/src/demangle/DemangleConfig.h
+++ b/libcxxabi/src/demangle/DemangleConfig.h
@@ -11,6 +11,11 @@
 #ifndef LIBCXXABI_DEMANGLE_DEMANGLE_CONFIG_H
 #define LIBCXXABI_DEMANGLE_DEMANGLE_CONFIG_H
 
+// Must be defined before pulling in headers from libc++.
+// https://libcxx.llvm.org/UsingLibcxx.html#enabling-the-safe-libc-mode
+#define _LIBCPP_VERBOSE_ABORT(...) abort_message(__VA_ARGS__)
+
+#include "../abort_message.h"
 #include <ciso646>
 
 #ifdef _MSC_VER