diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index d79091a1f6b6..26a80b51c852 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -84,6 +84,9 @@ def err_exponent_has_no_digits : Error<"exponent has no digits">;
 def ext_imaginary_constant : Extension<"imaginary constants are an extension">;
 def err_hexconstant_requires_exponent : Error<
   "hexadecimal floating constants require an exponent">;
+def ext_hexconstant_cplusplus : ExtWarn<
+  "hexadecimal floating constants are a C99 feature that is incompatible with "
+  "C++0x">;
 def ext_hexconstant_invalid : Extension<
   "hexadecimal floating constants are a C99 feature">;
 def ext_binary_literal : Extension<
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index d5a46433c36a..d82f8fcf8580 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -724,7 +724,8 @@ void Lexer::LexNumericConstant(Token &Result, const char *CurPtr) {
     return LexNumericConstant(Result, ConsumeChar(CurPtr, Size, Result));
 
   // If we have a hex FP constant, continue.
-  if ((C == '-' || C == '+') && (PrevCh == 'P' || PrevCh == 'p'))
+  if ((C == '-' || C == '+') && (PrevCh == 'P' || PrevCh == 'p') &&
+      (!PP || !PP->getLangOptions().CPlusPlus0x))
     return LexNumericConstant(Result, ConsumeChar(CurPtr, Size, Result));
 
   // Update the location of token as well as BufferPtr.
diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
index 9aaa82d6263c..5cd54975055d 100644
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -458,7 +458,7 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
     }
     // A binary exponent can appear with or with a '.'. If dotted, the
     // binary exponent is required.
-    if (*s == 'p' || *s == 'P') {
+    if ((*s == 'p' || *s == 'P') && !PP.getLangOptions().CPlusPlus0x) {
       const char *Exponent = s;
       s++;
       saw_exponent = true;
@@ -472,7 +472,12 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
       }
       s = first_non_digit;
 
-      if (!PP.getLangOptions().HexFloats)
+      // In C++0x, we cannot support hexadecmial floating literals because
+      // they conflict with user-defined literals, so we warn in previous
+      // versions of C++ by default.
+      if (PP.getLangOptions().CPlusPlus)
+        PP.Diag(TokLoc, diag::ext_hexconstant_cplusplus);
+      else if (!PP.getLangOptions().HexFloats)
         PP.Diag(TokLoc, diag::ext_hexconstant_invalid);
     } else if (saw_period) {
       PP.Diag(PP.AdvanceToTokenCharacter(TokLoc, s-ThisTokBegin),
diff --git a/clang/test/Lexer/hexfloat.cpp b/clang/test/Lexer/hexfloat.cpp
new file mode 100644
index 000000000000..5a62556ff616
--- /dev/null
+++ b/clang/test/Lexer/hexfloat.cpp
@@ -0,0 +1,8 @@
+//RUN: %clang_cc1 -fsyntax-only -verify
+//RUN: %clang_cc1 -std=c++0x -fsyntax-only -verify
+
+#ifndef __GXX_EXPERIMENTAL_CXX0X__
+float f = 0x1p+1; // expected-warning {{incompatible with C++0x}}
+#else
+float f = 0x1p+1; // expected-warning {{invalid suffix}}
+#endif