Index: compiler-rt/lib/fuzzer/CMakeLists.txt
===================================================================
--- compiler-rt/lib/fuzzer/CMakeLists.txt
+++ compiler-rt/lib/fuzzer/CMakeLists.txt
@@ -2,6 +2,7 @@
   FuzzerCrossOver.cpp
   FuzzerDataFlowTrace.cpp
   FuzzerDriver.cpp
+  FuzzerExtFunctionsAlternatename.cpp
   FuzzerExtFunctionsDlsym.cpp
   FuzzerExtFunctionsWeakAlias.cpp
   FuzzerExtFunctionsWeak.cpp
Index: compiler-rt/lib/fuzzer/FuzzerExtFunctionsAlternatename.cpp
===================================================================
--- /dev/null
+++ compiler-rt/lib/fuzzer/FuzzerExtFunctionsAlternatename.cpp
@@ -0,0 +1,73 @@
+//=== FuzzerExtFunctionsAlternatename.cpp - Interface to external functions ===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implementation using alternatenames. Works for MSVC on Windows.
+// Does not work with clang (see https://bugs.llvm.org/show_bug.cgi?id=40218#c0
+// for more details.
+//===----------------------------------------------------------------------===//
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS && LIBFUZZER_MSVC
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+using namespace fuzzer;
+
+extern "C" {
+// Declare these symbols as weak to allow them to default to a specified
+// function if not defined explicitly.
+// Copied from compiler-rt/lib/sanitizer_common/sanitizer_win_defs.h
+#if defined(_M_IX86) || defined(__i386__)
+#define WIN_SYM_PREFIX "_"
+#else
+#define WIN_SYM_PREFIX
+#endif
+
+// Intermediate macro to ensure the parameter is expanded before stringified.
+#define STRINGIFY_(A) #A
+#define STRINGIFY(A) STRINGIFY_(A)
+
+#define WIN_WEAK_ALIAS(Name, Default)                                  \
+  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY( \
+                               Name) "=" WIN_SYM_PREFIX STRINGIFY(Default)))
+
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)         \
+  RETURN_TYPE NAME##Def FUNC_SIG {                          \
+    Printf("ERROR: Function \"%s\" not defined.\n", #NAME); \
+    exit(1);                                                \
+  }                                                         \
+  WIN_WEAK_ALIAS(NAME, NAME##Def) RETURN_TYPE NAME FUNC_SIG;
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+template <typename T>
+static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
+  if (Fun == FunDef) {
+    if (WarnIfMissing)
+      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+    return nullptr;
+  }
+  return Fun;
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+} // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS && LIBFUZZER_MSVC
Index: compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
===================================================================
--- compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
+++ compiler-rt/lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
@@ -6,10 +6,10 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-// Implementation using weak aliases. Works for Windows.
+// Implementation using weak aliases. Works for clang on Windows.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#if LIBFUZZER_WINDOWS
+#if LIBFUZZER_WINDOWS && !LIBFUZZER_MSVC
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
@@ -53,4 +53,4 @@
 
 } // namespace fuzzer
 
-#endif // LIBFUZZER_WINDOWS
+#endif // LIBFUZZER_WINDOWS && !LIBFUZZER_MSVC