Index: include/llvm/ADT/FunctionExtras.h
===================================================================
--- include/llvm/ADT/FunctionExtras.h
+++ include/llvm/ADT/FunctionExtras.h
@@ -147,12 +147,19 @@
     StorageUnion.OutOfLineStorage = {Ptr, Size, Alignment};
   }
 
+#if defined(_MSC_VER) && defined(__clang__)
+  // The MS ABI does not align functions, but clang can do it.
+#define ALIGNED __attribute__((aligned(16)))
+#else
+#define ALIGNED
+#endif
   template <typename CallableT>
   static ReturnT CallImpl(void *CallableAddr,
-                          AdjustedParamT<ParamTs>... Params) {
+                          AdjustedParamT<ParamTs>... Params) ALIGNED {
     return (*reinterpret_cast<CallableT *>(CallableAddr))(
         std::forward<ParamTs>(Params)...);
   }
+#undef ALIGNED
 
   template <typename CallableT>
   static void MoveImpl(void *LHSCallableAddr, void *RHSCallableAddr) noexcept {
@@ -243,7 +250,7 @@
     // Now move into the storage.
     new (CallableAddr) CallableT(std::move(Callable));
 
-#ifndef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
     // See if we can create a trivial callback.
     //
     // This requires two things. First, we need to put a function pointer into
@@ -252,9 +259,6 @@
     // allows this to work, but on some platforms this appears to not hold, and
     // so we need to disable this optimization on those platforms.
     //
-    // FIXME: Currently, Windows appears to fail the asserts that check this.
-    // We should investigate why, and if fixed removed the #ifndef above.
-    //
     // Second, we need the callable to be trivially moved and trivially
     // destroyed so that we don't have to store type erased callbacks for those
     // operations.