Index: cfe/trunk/lib/Frontend/CompilerInstance.cpp
===================================================================
--- cfe/trunk/lib/Frontend/CompilerInstance.cpp
+++ cfe/trunk/lib/Frontend/CompilerInstance.cpp
@@ -232,9 +232,13 @@
                                                         std::move(StreamOwner));
   if (CodeGenOpts)
     Logger->setDwarfDebugFlags(CodeGenOpts->DwarfDebugFlags);
-  assert(Diags.ownsClient());
-  Diags.setClient(
-      new ChainedDiagnosticConsumer(Diags.takeClient(), std::move(Logger)));
+  if (Diags.ownsClient()) {
+    Diags.setClient(
+        new ChainedDiagnosticConsumer(Diags.takeClient(), std::move(Logger)));
+  } else {
+    Diags.setClient(
+        new ChainedDiagnosticConsumer(Diags.getClient(), std::move(Logger)));
+  }
 }
 
 static void SetupSerializedDiagnostics(DiagnosticOptions *DiagOpts,
Index: cfe/trunk/unittests/Frontend/CompilerInstanceTest.cpp
===================================================================
--- cfe/trunk/unittests/Frontend/CompilerInstanceTest.cpp
+++ cfe/trunk/unittests/Frontend/CompilerInstanceTest.cpp
@@ -8,6 +8,7 @@
 
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
+#include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/ToolOutputFile.h"
@@ -70,4 +71,21 @@
   ASSERT_TRUE(Instance.getFileManager().getFile("vfs-virtual.file"));
 }
 
+TEST(CompilerInstance, AllowDiagnosticLogWithUnownedDiagnosticConsumer) {
+  auto DiagOpts = new DiagnosticOptions();
+  DiagOpts->DiagnosticLogFile = "log.diags";
+
+  // Create the diagnostic engine with unowned consumer.
+  std::string DiagnosticOutput;
+  llvm::raw_string_ostream DiagnosticsOS(DiagnosticOutput);
+  auto DiagPrinter = llvm::make_unique<TextDiagnosticPrinter>(
+      DiagnosticsOS, new DiagnosticOptions());
+  CompilerInstance Instance;
+  IntrusiveRefCntPtr<DiagnosticsEngine> Diags = Instance.createDiagnostics(
+      DiagOpts, DiagPrinter.get(), /*ShouldOwnClient=*/false);
+
+  Diags->Report(diag::err_expected) << "no crash";
+  ASSERT_EQ(DiagnosticsOS.str(), "error: expected no crash\n");
+}
+
 } // anonymous namespace