Index: include/clang/Frontend/CompilerInvocation.h
===================================================================
--- include/clang/Frontend/CompilerInvocation.h
+++ include/clang/Frontend/CompilerInvocation.h
@@ -48,7 +48,8 @@
 /// report the error(s).
 bool ParseDiagnosticArgs(DiagnosticOptions &Opts, llvm::opt::ArgList &Args,
                          DiagnosticsEngine *Diags = nullptr,
-                         bool DefaultDiagColor = true);
+                         bool DefaultDiagColor = true,
+                         bool DefaultShowOpt = true);
 
 class CompilerInvocationBase : public RefCountedBase<CompilerInvocation> {
   void operator=(const CompilerInvocationBase &) = delete;
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -941,7 +941,7 @@
 
 bool clang::ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
                                 DiagnosticsEngine *Diags,
-                                bool DefaultDiagColor) {
+                                bool DefaultDiagColor, bool DefaultShowOpt) {
   using namespace options;
   bool Success = true;
 
@@ -961,7 +961,8 @@
   Opts.ShowFixits = !Args.hasArg(OPT_fno_diagnostics_fixit_info);
   Opts.ShowLocation = !Args.hasArg(OPT_fno_show_source_location);
   Opts.AbsolutePath = Args.hasArg(OPT_fdiagnostics_absolute_paths);
-  Opts.ShowOptionNames = Args.hasArg(OPT_fdiagnostics_show_option);
+  Opts.ShowOptionNames = Args.hasArg(OPT_fdiagnostics_show_option) ||
+      (DefaultShowOpt && !Args.hasArg(OPT_fno_diagnostics_show_option));
 
   llvm::sys::Process::UseANSIEscapeCodes(Args.hasArg(OPT_fansi_escape_codes));
 
@@ -2373,8 +2374,9 @@
   Success &= ParseAnalyzerArgs(*Res.getAnalyzerOpts(), Args, Diags);
   Success &= ParseMigratorArgs(Res.getMigratorOpts(), Args);
   ParseDependencyOutputArgs(Res.getDependencyOutputOpts(), Args);
-  Success &= ParseDiagnosticArgs(Res.getDiagnosticOpts(), Args, &Diags,
-                                 false /*DefaultDiagColor*/);
+  Success &=
+      ParseDiagnosticArgs(Res.getDiagnosticOpts(), Args, &Diags,
+                          false /*DefaultDiagColor*/, false /*DefaultShowOpt*/);
   ParseCommentArgs(LangOpts.CommentOpts, Args);
   ParseFileSystemArgs(Res.getFileSystemOpts(), Args);
   // FIXME: We shouldn't have to pass the DashX option around here
Index: test/Driver/show-option-names.c
===================================================================
--- /dev/null
+++ test/Driver/show-option-names.c
@@ -0,0 +1,4 @@
+// RUN: %clang -c -target i386-apple-darwin10 -isysroot /FOO %s 2>&1 | FileCheck --check-prefix=CHECK-SHOW-OPTION-NAMES %s
+// CHECK-SHOW-OPTION-NAMES: warning: no such sysroot directory: '{{([A-Za-z]:.*)?}}/FOO' [-Wmissing-sysroot]
+// RUN: %clang -c -target i386-apple-darwin10 -fno-diagnostics-show-option -isysroot /FOO %s 2>&1 | FileCheck --check-prefix=CHECK-NO-SHOW-OPTION-NAMES %s
+// CHECK-NO-SHOW-OPTION-NAMES-NOT: warning: no such sysroot directory: '{{([A-Za-z]:.*)?}}/FOO' [-Wmissing-sysroot]