Index: cfe/trunk/include/clang/Basic/SourceManager.h
===================================================================
--- cfe/trunk/include/clang/Basic/SourceManager.h
+++ cfe/trunk/include/clang/Basic/SourceManager.h
@@ -1440,6 +1440,12 @@
     return Filename.equals("<command line>");
   }
 
+  /// Returns whether \p Loc is located in a <scratch space> file.
+  bool isWrittenInScratchSpace(SourceLocation Loc) const {
+    StringRef Filename(getPresumedLoc(Loc).getFilename());
+    return Filename.equals("<scratch space>");
+  }
+
   /// Returns if a SourceLocation is in a system header.
   bool isInSystemHeader(SourceLocation Loc) const {
     return isSystem(getFileCharacteristic(Loc));
@@ -1452,7 +1458,17 @@
 
   /// Returns whether \p Loc is expanded from a macro in a system header.
   bool isInSystemMacro(SourceLocation loc) const {
-    return loc.isMacroID() && isInSystemHeader(getSpellingLoc(loc));
+    if(!loc.isMacroID())
+      return false;
+
+    // This happens when the macro is the result of a paste, in that case
+    // its spelling is the scratch memory, so we take the parent context.
+    if (isWrittenInScratchSpace(getSpellingLoc(loc))) {
+      return isInSystemHeader(getSpellingLoc(getImmediateMacroCallerLoc(loc)));
+    }
+    else {
+      return isInSystemHeader(getSpellingLoc(loc));
+    }
   }
 
   /// The size of the SLocEntry that \p FID represents.
Index: cfe/trunk/test/Misc/no-warn-in-system-macro.c
===================================================================
--- cfe/trunk/test/Misc/no-warn-in-system-macro.c
+++ cfe/trunk/test/Misc/no-warn-in-system-macro.c
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -isystem %S -Wdouble-promotion -fsyntax-only %s  2>&1 | FileCheck -allow-empty %s
+// CHECK-NOT: warning:
+
+#include <no-warn-in-system-macro.c.inc>
+
+int main(void)
+{
+	double foo = 1.0;
+
+	if (isnan(foo))
+		return 1;
+	return 0;
+}
Index: cfe/trunk/test/Misc/no-warn-in-system-macro.c.inc
===================================================================
--- cfe/trunk/test/Misc/no-warn-in-system-macro.c.inc
+++ cfe/trunk/test/Misc/no-warn-in-system-macro.c.inc
@@ -0,0 +1,9 @@
+extern int __isnanf(float f);
+extern int __isnan(double f);
+extern int __isnanl(long double f);
+#define isnan(x) \
+	(sizeof (x) == sizeof (float)                \
+	? __isnanf (x)                    \
+	: sizeof (x) == sizeof (double)               \
+	? __isnan (x) : __isnanl (x))
+
Index: cfe/trunk/test/Misc/warn-in-system-macro-def.c
===================================================================
--- cfe/trunk/test/Misc/warn-in-system-macro-def.c
+++ cfe/trunk/test/Misc/warn-in-system-macro-def.c
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -isystem %S -Wdouble-promotion -fsyntax-only %s  2>&1 | FileCheck -allow-empty %s
+// CHECK: warning:
+// CHECK: expanded from macro 'ISNAN'
+// CHECK: expanded from macro 'isnan'
+
+#include <warn-in-system-macro-def.c.inc>
+
+#define isnan(x) \
+	(sizeof (x) == sizeof (float)                \
+	? __isnanf (x)                    \
+	: sizeof (x) == sizeof (double)               \
+	? __isnan (x) : __isnanl (x))
+
+int main(void)
+{
+	double foo = 1.0;
+
+	if (ISNAN(foo))
+		return 1;
+	return 0;
+}
Index: cfe/trunk/test/Misc/warn-in-system-macro-def.c.inc
===================================================================
--- cfe/trunk/test/Misc/warn-in-system-macro-def.c.inc
+++ cfe/trunk/test/Misc/warn-in-system-macro-def.c.inc
@@ -0,0 +1,4 @@
+extern int __isnanf(float f);
+extern int __isnan(double f);
+extern int __isnanl(long double f);
+#define ISNAN isnan