diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -830,7 +830,7 @@
 
 static bool parseDiagnosticLevelMask(StringRef FlagName,
                                      const std::vector<std::string> &Levels,
-                                     DiagnosticsEngine *Diags,
+                                     DiagnosticsEngine &Diags,
                                      DiagnosticLevelMask &M) {
   bool Success = true;
   for (const auto &Level : Levels) {
@@ -843,8 +843,7 @@
         .Default(DiagnosticLevelMask::None);
     if (PM == DiagnosticLevelMask::None) {
       Success = false;
-      if (Diags)
-        Diags->Report(diag::err_drv_invalid_value) << FlagName << Level;
+      Diags.Report(diag::err_drv_invalid_value) << FlagName << Level;
     }
     M = M | PM;
   }
@@ -1383,7 +1382,7 @@
 }
 
 static bool checkVerifyPrefixes(const std::vector<std::string> &VerifyPrefixes,
-                                DiagnosticsEngine *Diags) {
+                                DiagnosticsEngine &Diags) {
   bool Success = true;
   for (const auto &Prefix : VerifyPrefixes) {
     // Every prefix must start with a letter and contain only alphanumeric
@@ -1393,10 +1392,8 @@
     });
     if (BadChar != Prefix.end() || !isLetter(Prefix[0])) {
       Success = false;
-      if (Diags) {
-        Diags->Report(diag::err_drv_invalid_value) << "-verify=" << Prefix;
-        Diags->Report(diag::note_drv_verify_prefix_spelling);
-      }
+      Diags.Report(diag::err_drv_invalid_value) << "-verify=" << Prefix;
+      Diags.Report(diag::note_drv_verify_prefix_spelling);
     }
   }
   return Success;
@@ -1405,6 +1402,13 @@
 bool clang::ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
                                 DiagnosticsEngine *Diags,
                                 bool DefaultDiagColor) {
+  Optional<DiagnosticsEngine> IgnoringDiags;
+  if (!Diags) {
+    IgnoringDiags.emplace(new DiagnosticIDs(), new DiagnosticOptions(),
+                          new IgnoringDiagConsumer());
+    Diags = &*IgnoringDiags;
+  }
+
   bool Success = true;
 
   Opts.DiagnosticLogFile =
@@ -1439,10 +1443,9 @@
     Opts.setShowOverloads(Ovl_All);
   else {
     Success = false;
-    if (Diags)
-      Diags->Report(diag::err_drv_invalid_value)
-      << Args.getLastArg(OPT_fshow_overloads_EQ)->getAsString(Args)
-      << ShowOverloads;
+    Diags->Report(diag::err_drv_invalid_value)
+        << Args.getLastArg(OPT_fshow_overloads_EQ)->getAsString(Args)
+        << ShowOverloads;
   }
 
   StringRef ShowCategory =
@@ -1455,10 +1458,9 @@
     Opts.ShowCategories = 2;
   else {
     Success = false;
-    if (Diags)
-      Diags->Report(diag::err_drv_invalid_value)
-      << Args.getLastArg(OPT_fdiagnostics_show_category)->getAsString(Args)
-      << ShowCategory;
+    Diags->Report(diag::err_drv_invalid_value)
+        << Args.getLastArg(OPT_fdiagnostics_show_category)->getAsString(Args)
+        << ShowCategory;
   }
 
   StringRef Format =
@@ -1474,10 +1476,9 @@
     Opts.setFormat(DiagnosticOptions::Vi);
   else {
     Success = false;
-    if (Diags)
-      Diags->Report(diag::err_drv_invalid_value)
-      << Args.getLastArg(OPT_fdiagnostics_format)->getAsString(Args)
-      << Format;
+    Diags->Report(diag::err_drv_invalid_value)
+        << Args.getLastArg(OPT_fdiagnostics_format)->getAsString(Args)
+        << Format;
   }
 
   Opts.ShowSourceRanges = Args.hasArg(OPT_fdiagnostics_print_source_range_info);
@@ -1488,7 +1489,7 @@
     Opts.VerifyPrefixes.push_back("expected");
   // Keep VerifyPrefixes in its original order for the sake of diagnostics, and
   // then sort it to prepare for fast lookup using std::binary_search.
-  if (!checkVerifyPrefixes(Opts.VerifyPrefixes, Diags)) {
+  if (!checkVerifyPrefixes(Opts.VerifyPrefixes, *Diags)) {
     Opts.VerifyDiagnostics = false;
     Success = false;
   }
@@ -1497,7 +1498,7 @@
   DiagnosticLevelMask DiagMask = DiagnosticLevelMask::None;
   Success &= parseDiagnosticLevelMask("-verify-ignore-unexpected=",
     Args.getAllArgValues(OPT_verify_ignore_unexpected_EQ),
-    Diags, DiagMask);
+    *Diags, DiagMask);
   if (Args.hasArg(OPT_verify_ignore_unexpected))
     DiagMask = DiagnosticLevelMask::All;
   Opts.setVerifyIgnoreUnexpected(DiagMask);
@@ -1523,9 +1524,8 @@
                                     DiagnosticOptions::DefaultTabStop, Diags);
   if (Opts.TabStop == 0 || Opts.TabStop > DiagnosticOptions::MaxTabStop) {
     Opts.TabStop = DiagnosticOptions::DefaultTabStop;
-    if (Diags)
-      Diags->Report(diag::warn_ignoring_ftabstop_value)
-      << Opts.TabStop << DiagnosticOptions::DefaultTabStop;
+    Diags->Report(diag::warn_ignoring_ftabstop_value)
+        << Opts.TabStop << DiagnosticOptions::DefaultTabStop;
   }
   Opts.MessageLength =
       getLastArgIntValue(Args, OPT_fmessage_length_EQ, 0, Diags);