diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index b0e784b..4803c5a 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1514,18 +1514,21 @@
   // Check to see if this is the last token on the #if[n]def line.
   CheckEndOfDirective(isIfndef ? "ifndef" : "ifdef");
 
+  IdentifierInfo *MII = MacroNameTok.getIdentifierInfo();
+  MacroInfo *MI = getMacroInfo(MII);
+  
   if (CurPPLexer->getConditionalStackDepth() == 0) {
-    // If the start of a top-level #ifdef, inform MIOpt.
-    if (!ReadAnyTokensBeforeDirective) {
+    // If the start of a top-level #ifdef and if the macro is not defined,
+    // inform MIOpt that this might be the start of a proper include guard.
+    // Otherwise it is some other form of unknown conditional which we can't
+    // handle.
+    if (!ReadAnyTokensBeforeDirective && MI == 0) {
       assert(isIfndef && "#ifdef shouldn't reach here");
-      CurPPLexer->MIOpt.EnterTopLevelIFNDEF(MacroNameTok.getIdentifierInfo());
+      CurPPLexer->MIOpt.EnterTopLevelIFNDEF(MII);
     } else
       CurPPLexer->MIOpt.EnterTopLevelConditional();
   }
 
-  IdentifierInfo *MII = MacroNameTok.getIdentifierInfo();
-  MacroInfo *MI = getMacroInfo(MII);
-
   // If there is a macro, process it.
   if (MI)  // Mark it used.
     MI->setIsUsed(true);
@@ -1558,7 +1561,7 @@
   // If this condition is equivalent to #ifndef X, and if this is the first
   // directive seen, handle it for the multiple-include optimization.
   if (CurPPLexer->getConditionalStackDepth() == 0) {
-    if (!ReadAnyTokensBeforeDirective && IfNDefMacro)
+    if (!ReadAnyTokensBeforeDirective && IfNDefMacro && ConditionalTrue)
       CurPPLexer->MIOpt.EnterTopLevelIFNDEF(IfNDefMacro);
     else
       CurPPLexer->MIOpt.EnterTopLevelConditional();
diff --git a/clang/test/Preprocessor/mi_opt2.c b/clang/test/Preprocessor/mi_opt2.c
new file mode 100644
index 0000000..198d19f
--- /dev/null
+++ b/clang/test/Preprocessor/mi_opt2.c
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -E %s | FileCheck %s
+// PR6282
+// This test should not trigger the include guard optimization since
+// the guard macro is defined on the first include.
+
+#define ITERATING 1
+#define X 1
+#include "mi_opt2.h"
+#undef X
+#define X 2
+#include "mi_opt2.h"
+
+// CHECK: b: 1
+// CHECK: b: 2
+
diff --git a/clang/test/Preprocessor/mi_opt2.h b/clang/test/Preprocessor/mi_opt2.h
new file mode 100644
index 0000000..df37eba
--- /dev/null
+++ b/clang/test/Preprocessor/mi_opt2.h
@@ -0,0 +1,5 @@
+#ifndef ITERATING
+a: X
+#else
+b: X
+#endif