Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -505,8 +505,6 @@
 def err_pragma_message : Error<"%0">;
 def warn_pragma_ignored : Warning<"unknown pragma ignored">,
    InGroup<UnknownPragmas>, DefaultIgnore;
-def ext_stdc_pragma_ignored : ExtWarn<"unknown pragma in STDC namespace">,
-   InGroup<UnknownPragmas>;
 def ext_on_off_switch_syntax :
    ExtWarn<"expected 'ON' or 'OFF' or 'DEFAULT' in pragma">,
    InGroup<UnknownPragmas>;
Index: include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- include/clang/Basic/DiagnosticParseKinds.td
+++ include/clang/Basic/DiagnosticParseKinds.td
@@ -973,6 +973,9 @@
 def err_pragma_fp_contract_scope : Error<
   "'#pragma fp_contract' can only appear at file scope or at the start of a "
   "compound statement">; 
+// - #pragma stdc unknown
+def ext_stdc_pragma_ignored : ExtWarn<"unknown pragma in STDC namespace">,
+   InGroup<UnknownPragmas>;
 // - #pragma comment
 def err_pragma_comment_malformed : Error<
   "pragma comment requires parenthesized identifier and optional string">;
Index: include/clang/Parse/Parser.h
===================================================================
--- include/clang/Parse/Parser.h
+++ include/clang/Parse/Parser.h
@@ -185,6 +185,7 @@
   std::unique_ptr<PragmaHandler> UnrollHintHandler;
   std::unique_ptr<PragmaHandler> NoUnrollHintHandler;
   std::unique_ptr<PragmaHandler> FPHandler;
+  std::unique_ptr<PragmaHandler> STDCUnknownHandler;
   std::unique_ptr<PragmaHandler> AttributePragmaHandler;
 
   std::unique_ptr<CommentHandler> CommentSemaHandler;
Index: lib/Frontend/PrintPreprocessedOutput.cpp
===================================================================
--- lib/Frontend/PrintPreprocessedOutput.cpp
+++ lib/Frontend/PrintPreprocessedOutput.cpp
@@ -851,9 +851,14 @@
       "#pragma clang", Callbacks,
       /*RequireTokenExpansion=*/PP.getLangOpts().MicrosoftExt));
 
+  std::unique_ptr<UnknownPragmaHandler> STDCHandler(new UnknownPragmaHandler(
+      "#pragma STDC", Callbacks,
+      /*RequireTokenExpansion=*/PP.getLangOpts().MicrosoftExt));
+
   PP.AddPragmaHandler(MicrosoftExtHandler.get());
   PP.AddPragmaHandler("GCC", GCCHandler.get());
   PP.AddPragmaHandler("clang", ClangHandler.get());
+  PP.AddPragmaHandler("STDC", STDCHandler.get());
 
   // The tokens after pragma omp need to be expanded.
   //
@@ -897,5 +902,6 @@
   PP.RemovePragmaHandler(MicrosoftExtHandler.get());
   PP.RemovePragmaHandler("GCC", GCCHandler.get());
   PP.RemovePragmaHandler("clang", ClangHandler.get());
+  PP.RemovePragmaHandler("STDC", STDCHandler.get());
   PP.RemovePragmaHandler("omp", OpenMPHandler.get());
 }
Index: lib/Lex/Pragma.cpp
===================================================================
--- lib/Lex/Pragma.cpp
+++ lib/Lex/Pragma.cpp
@@ -1628,17 +1628,6 @@
   }
 };
 
-/// PragmaSTDC_UnknownHandler - "\#pragma STDC ...".
-struct PragmaSTDC_UnknownHandler : public PragmaHandler {
-  PragmaSTDC_UnknownHandler() = default;
-
-  void HandlePragma(Preprocessor &PP, PragmaIntroducerKind Introducer,
-                    Token &UnknownTok) override {
-    // C99 6.10.6p2, unknown forms are not allowed.
-    PP.Diag(UnknownTok, diag::ext_stdc_pragma_ignored);
-  }
-};
-
 /// PragmaARCCFCodeAuditedHandler - 
 ///   \#pragma clang arc_cf_code_audited begin/end
 struct PragmaARCCFCodeAuditedHandler : public PragmaHandler {
@@ -1815,9 +1804,9 @@
   ModuleHandler->AddPragma(new PragmaModuleBuildHandler());
   ModuleHandler->AddPragma(new PragmaModuleLoadHandler());
 
+  // #pragma STDC
   AddPragmaHandler("STDC", new PragmaSTDC_FENV_ACCESSHandler());
   AddPragmaHandler("STDC", new PragmaSTDC_CX_LIMITED_RANGEHandler());
-  AddPragmaHandler("STDC", new PragmaSTDC_UnknownHandler());
 
   // MS extensions.
   if (LangOpts.MicrosoftExt) {
@@ -1843,17 +1832,5 @@
   // in Preprocessor::RegisterBuiltinPragmas().
   AddPragmaHandler("GCC", new EmptyPragmaHandler());
   AddPragmaHandler("clang", new EmptyPragmaHandler());
-  if (PragmaHandler *NS = PragmaHandlers->FindHandler("STDC")) {
-    // Preprocessor::RegisterBuiltinPragmas() already registers
-    // PragmaSTDC_UnknownHandler as the empty handler, so remove it first,
-    // otherwise there will be an assert about a duplicate handler.
-    PragmaNamespace *STDCNamespace = NS->getIfNamespace();
-    assert(STDCNamespace &&
-           "Invalid namespace, registered as a regular pragma handler!");
-    if (PragmaHandler *Existing = STDCNamespace->FindHandler("", false)) {
-      RemovePragmaHandler("STDC", Existing);
-      delete Existing;
-    }
-  }
   AddPragmaHandler("STDC", new EmptyPragmaHandler());
 }
Index: lib/Parse/ParsePragma.cpp
===================================================================
--- lib/Parse/ParsePragma.cpp
+++ lib/Parse/ParsePragma.cpp
@@ -95,6 +95,17 @@
                     Token &FirstToken) override;
 };
 
+/// PragmaSTDC_UnknownHandler - "\#pragma STDC ...".
+struct PragmaSTDC_UnknownHandler : public PragmaHandler {
+  PragmaSTDC_UnknownHandler() = default;
+
+  void HandlePragma(Preprocessor &PP, PragmaIntroducerKind Introducer,
+                    Token &UnknownTok) override {
+    // C99 6.10.6p2, unknown forms are not allowed.
+    PP.Diag(UnknownTok, diag::ext_stdc_pragma_ignored);
+  }
+};
+
 struct PragmaFPHandler : public PragmaHandler {
   PragmaFPHandler() : PragmaHandler("fp") {}
   void HandlePragma(Preprocessor &PP, PragmaIntroducerKind Introducer,
@@ -233,6 +244,9 @@
   FPContractHandler.reset(new PragmaFPContractHandler());
   PP.AddPragmaHandler("STDC", FPContractHandler.get());
 
+  STDCUnknownHandler.reset(new PragmaSTDC_UnknownHandler());
+  PP.AddPragmaHandler("STDC", STDCUnknownHandler.get());
+
   PCSectionHandler.reset(new PragmaClangSectionHandler(Actions));
   PP.AddPragmaHandler("clang", PCSectionHandler.get());
 
@@ -371,6 +385,9 @@
   PP.RemovePragmaHandler("STDC", FPContractHandler.get());
   FPContractHandler.reset();
 
+  PP.RemovePragmaHandler("STDC", STDCUnknownHandler.get());
+  STDCUnknownHandler.reset();
+
   PP.RemovePragmaHandler("clang", OptimizeHandler.get());
   OptimizeHandler.reset();
 
Index: test/Preprocessor/pragma_unknown.c
===================================================================
--- test/Preprocessor/pragma_unknown.c
+++ test/Preprocessor/pragma_unknown.c
@@ -1,14 +1,18 @@
 // RUN: %clang_cc1 -fsyntax-only -Wunknown-pragmas -verify %s
-// RUN: %clang_cc1 -E %s | FileCheck --strict-whitespace %s
+// RUN: %clang_cc1 -E %s 2>&1 | FileCheck --strict-whitespace %s
 
 // GCC doesn't expand macro args for unrecognized pragmas.
 #define bar xX
 #pragma foo bar   // expected-warning {{unknown pragma ignored}}
+// CHECK-NOT: unknown pragma in STDC namespace
 // CHECK: {{^}}#pragma foo bar{{$}}
 
 #pragma STDC FP_CONTRACT ON
 #pragma STDC FP_CONTRACT OFF
 #pragma STDC FP_CONTRACT DEFAULT
+// CHECK: {{^}}#pragma STDC FP_CONTRACT ON{{$}}
+// CHECK: {{^}}#pragma STDC FP_CONTRACT OFF{{$}}
+// CHECK: {{^}}#pragma STDC FP_CONTRACT DEFAULT{{$}}
 #pragma STDC FP_CONTRACT IN_BETWEEN  // expected-warning {{expected 'ON' or 'OFF' or 'DEFAULT' in pragma}}
 
 #pragma STDC FENV_ACCESS ON          // expected-warning {{pragma STDC FENV_ACCESS ON is not supported, ignoring pragma}}