Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -2727,7 +2727,9 @@
                              /*Syntactic=*/LangOpts.MicrosoftExt))
       Diag(MI->getDefinitionLoc(), diag::warn_pp_macro_def_mismatch_with_pch)
           << MacroNameTok.getIdentifierInfo();
-    return;
+    // Issue the diagnostic but allow the change if msvc extensions are enabled
+    if (!LangOpts.MicrosoftExt)
+      return;
   }
 
   // Finally, if this identifier already had a macro defined for it, verify that
Index: clang/test/PCH/fuzzy-pch-msvc.c
===================================================================
--- /dev/null
+++ clang/test/PCH/fuzzy-pch-msvc.c
@@ -0,0 +1,32 @@
+// Test -D and -U interaction with a PCH when running clang-cl.
+
+// RUN: %clang_cl -DFOO -Yc %S/variables.h
+
+// RUN: not %clang_cl -c -DFOO=blah -DBAR=int -Wno-microsoft-include -Yuvariables.h %s 2> %t.err
+// RUN: FileCheck -check-prefix=CHECK-FOO %s < %t.err
+
+// RUN: not %clang_cl -c -UFOO -DBAR=int -Wno-microsoft-include -Yuvariables.h %s 2> %t.err
+// RUN: FileCheck -check-prefix=CHECK-NOFOO %s < %t.err 
+
+// RUN: %clang_cl -c -DBAR=int -Wno-microsoft-include -Yuvariables.h -Xclang -verify %s 
+#include "variables.h"
+
+BAR bar = 17;
+
+#ifndef FOO
+#  error FOO was not defined
+#endif
+
+#if FOO != 1
+#  error FOO has the wrong definition
+#endif
+
+#ifndef BAR
+#  error BAR was not defined 
+#endif
+
+
+// CHECK-FOO: definition of macro 'FOO' differs between the precompiled header ('1') and the command line ('blah')
+// CHECK-NOFOO: macro 'FOO' was defined in the precompiled header but undef'd on the command line
+
+// expected-warning@1 {{definition of macro 'BAR' does not match definition in precompiled header}}