Index: lib/tsan/rtl/tsan_debugging.cc
===================================================================
--- lib/tsan/rtl/tsan_debugging.cc
+++ lib/tsan/rtl/tsan_debugging.cc
@@ -25,6 +25,7 @@
   if (typ == ReportTypeUseAfterFree) return "heap-use-after-free";
   if (typ == ReportTypeVptrUseAfterFree) return "heap-use-after-free-vptr";
   if (typ == ReportTypeExternalRace) return "external-race";
+  if (typ == ReportTypeSwiftAccessRace) return "swift-access-race";
   if (typ == ReportTypeThreadLeak) return "thread-leak";
   if (typ == ReportTypeMutexDestroyLocked) return "locked-mutex-destroy";
   if (typ == ReportTypeMutexDoubleLock) return "mutex-double-lock";
Index: lib/tsan/rtl/tsan_report.h
===================================================================
--- lib/tsan/rtl/tsan_report.h
+++ lib/tsan/rtl/tsan_report.h
@@ -25,6 +25,7 @@
   ReportTypeUseAfterFree,
   ReportTypeVptrUseAfterFree,
   ReportTypeExternalRace,
+  ReportTypeSwiftAccessRace,
   ReportTypeThreadLeak,
   ReportTypeMutexDestroyLocked,
   ReportTypeMutexDoubleLock,
Index: lib/tsan/rtl/tsan_report.cc
===================================================================
--- lib/tsan/rtl/tsan_report.cc
+++ lib/tsan/rtl/tsan_report.cc
@@ -92,6 +92,8 @@
     return "heap-use-after-free (virtual call vs free)";
   if (typ == ReportTypeExternalRace)
     return "race on a library object";
+  if (typ == ReportTypeSwiftAccessRace)
+    return "Swift access race";
   if (typ == ReportTypeThreadLeak)
     return "thread leak";
   if (typ == ReportTypeMutexDestroyLocked)
@@ -159,12 +161,20 @@
                : (write ? "Previous mutating" : "Previous read-only");
 }
 
+static const char *SwiftMopDesc(bool first) {
+  return first ? "Modifying" : "Previous modifying";
+}
+
 static void PrintMop(const ReportMop *mop, bool first) {
   Decorator d;
   char thrbuf[kThreadBufSize];
   Printf("%s", d.Access());
   const char *object_type = GetObjectTypeFromTag(mop->external_tag);
-  if (!object_type) {
+  if (mop->external_tag == kExternalTagSwiftModifyingAccess) {
+    Printf("  %s access at %p by %s",
+           SwiftMopDesc(first), (void *)mop->addr,
+           thread_name(thrbuf, mop->tid));
+  } else if (mop->external_tag == kExternalTagNone || !object_type) {
     Printf("  %s of size %d at %p by %s",
            MopDesc(first, mop->write, mop->atomic), mop->size,
            (void *)mop->addr, thread_name(thrbuf, mop->tid));
Index: lib/tsan/rtl/tsan_rtl_report.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl_report.cc
+++ lib/tsan/rtl/tsan_rtl_report.cc
@@ -654,9 +654,15 @@
   ThreadRegistryLock l0(ctx->thread_registry);
   ScopedReport rep(typ);
   for (uptr i = 0; i < kMop; i++) {
+    uptr tag = RetrieveTagFromStackTrace(&traces[i]);
+    if (tag == kExternalTagSwiftModifyingAccess) {
+      rep.SetType(ReportTypeSwiftAccessRace);
+    } else if (tag != kExternalTagNone) {
+      rep.SetType(ReportTypeExternalRace);
+    }
+
     Shadow s(thr->racy_state[i]);
-    rep.AddMemoryAccess(addr, thr->external_tag, s, traces[i],
-                        i == 0 ? &thr->mset : mset2);
+    rep.AddMemoryAccess(addr, tag, s, traces[i], i == 0 ? &thr->mset : mset2);
   }
 
   for (uptr i = 0; i < kMop; i++) {
Index: lib/tsan/rtl/tsan_suppressions.cc
===================================================================
--- lib/tsan/rtl/tsan_suppressions.cc
+++ lib/tsan/rtl/tsan_suppressions.cc
@@ -76,6 +76,8 @@
     return kSuppressionRace;
   else if (typ == ReportTypeExternalRace)
     return kSuppressionRace;
+  else if (typ == ReportTypeSwiftAccessRace)
+    return kSuppressionRace;
   else if (typ == ReportTypeThreadLeak)
     return kSuppressionThread;
   else if (typ == ReportTypeMutexDestroyLocked)
Index: test/tsan/Darwin/external-swift.cc
===================================================================
--- test/tsan/Darwin/external-swift.cc
+++ test/tsan/Darwin/external-swift.cc
@@ -0,0 +1,85 @@
+// RUN: %clangxx_tsan %s -o %t
+// RUN: %deflake %run %t 2>&1 | FileCheck %s
+
+#include <thread>
+
+#import "../test.h"
+
+static void *tag = (void *)0x1;
+
+__attribute__((no_sanitize("thread")))
+void ExternalWrite(void *addr) {
+  __tsan_external_write(addr, nullptr, tag);
+}
+
+__attribute__((no_sanitize("thread")))
+void RegularWrite(void *addr) {
+  __tsan_write8(addr);
+}
+
+int main(int argc, char *argv[]) {
+  barrier_init(&barrier, 2);
+  fprintf(stderr, "Start.\n");
+  // CHECK: Start.
+  
+  {
+    void *opaque_object = malloc(16);
+    std::thread t1([opaque_object] {
+      ExternalWrite(opaque_object);
+      barrier_wait(&barrier);
+    });
+    std::thread t2([opaque_object] {
+      barrier_wait(&barrier);
+      ExternalWrite(opaque_object);
+    });
+    // CHECK: WARNING: ThreadSanitizer: Swift access race
+    // CHECK: Modifying access at {{.*}} by thread {{.*}}
+    // CHECK: Previous modifying access at {{.*}} by thread {{.*}}
+    t1.join();
+    t2.join();
+  }
+
+  fprintf(stderr, "external+external test done.\n");
+  // CHECK: external+external test done.
+
+  {
+    void *opaque_object = malloc(16);
+    std::thread t1([opaque_object] {
+      ExternalWrite(opaque_object);
+      barrier_wait(&barrier);
+    });
+    std::thread t2([opaque_object] {
+      barrier_wait(&barrier);
+      RegularWrite(opaque_object);
+    });
+    // CHECK: WARNING: ThreadSanitizer: Swift access race
+    // CHECK: Write of size 8 at {{.*}} by thread {{.*}}
+    // CHECK: Previous modifying access at {{.*}} by thread {{.*}}
+    t1.join();
+    t2.join();
+  }
+  
+  fprintf(stderr, "external+regular test done.\n");
+  // CHECK: external+regular test done.
+  
+  {
+    void *opaque_object = malloc(16);
+    std::thread t1([opaque_object] {
+      RegularWrite(opaque_object);
+      barrier_wait(&barrier);
+    });
+    std::thread t2([opaque_object] {
+      barrier_wait(&barrier);
+      ExternalWrite(opaque_object);
+    });
+    // CHECK: WARNING: ThreadSanitizer: Swift access race
+    // CHECK: Modifying access at {{.*}} by thread {{.*}}
+    // CHECK: Previous write of size 8 at {{.*}} by thread {{.*}}
+    t1.join();
+    t2.join();
+  }
+  
+  fprintf(stderr, "regular+external test done.\n");
+  // CHECK: regular+external test done.
+}
+