Index: lib/Lex/TokenLexer.cpp
===================================================================
--- lib/Lex/TokenLexer.cpp
+++ lib/Lex/TokenLexer.cpp
@@ -183,6 +183,12 @@
     // preprocessor already verified that the following token is a macro name
     // when the #define was parsed.
     const Token &CurTok = Tokens[i];
+    // We don't want a space for the next token after a paste
+    // operator.  In valid code, the token will get smooshed onto the
+    // preceding one anyway. In assembler-with-cpp mode, invalid
+    // pastes are allowed through: in this case, we do not want the
+    // extra whitespace to be added.  For example, we want ". ## foo"
+    // -> ".foo" not ". foo".
     if (i != 0 && !Tokens[i-1].is(tok::hashhash) && CurTok.hasLeadingSpace())
       NextTokGetsSpace = true;
 
@@ -317,6 +323,7 @@
     const Token *ArgToks = ActualArgs->getUnexpArgument(ArgNo);
     unsigned NumToks = MacroArgs::getArgLength(ArgToks);
     if (NumToks) {  // Not an empty argument?
+      bool VaArgsPseudoPaste = false;
       // If this is the GNU ", ## __VA_ARGS__" extension, and we just learned
       // that __VA_ARGS__ expands to multiple tokens, avoid a pasting error when
       // the expander trys to paste ',' with the first token of the __VA_ARGS__
@@ -325,6 +332,7 @@
           ResultToks[ResultToks.size()-2].is(tok::comma) &&
           (unsigned)ArgNo == Macro->getNumArgs()-1 &&
           Macro->isVariadic()) {
+        VaArgsPseudoPaste = true;
         // Remove the paste operator, report use of the extension.
         PP.Diag(ResultToks.pop_back_val().getLocation(), diag::ext_paste_comma);
       }
@@ -344,18 +352,16 @@
                                    ResultToks.end()-NumToks, ResultToks.end());
       }
 
-      // If this token (the macro argument) was supposed to get leading
-      // whitespace, transfer this information onto the first token of the
-      // expansion.
-      //
-      // Do not do this if the paste operator occurs before the macro argument,
-      // as in "A ## MACROARG".  In valid code, the first token will get
-      // smooshed onto the preceding one anyway (forming AMACROARG).  In
-      // assembler-with-cpp mode, invalid pastes are allowed through: in this
-      // case, we do not want the extra whitespace to be added.  For example,
-      // we want ". ## foo" -> ".foo" not ". foo".
-      if (NextTokGetsSpace)
-        ResultToks[ResultToks.size()-NumToks].setFlag(Token::LeadingSpace);
+      // Transfer the leading whitespace information from the token
+      // (the macro argument) onto the first token of the
+      // expansion. Note that we don't do this for the GNU
+      // pseudo-paste extension ", ## __VA_ARGS__".
+      if (!VaArgsPseudoPaste) {
+        ResultToks[ResultToks.size()-NumToks].setFlagValue(Token::StartOfLine,
+                                                           false);
+        ResultToks[ResultToks.size()-NumToks].setFlagValue(Token::LeadingSpace,
+                                                           NextTokGetsSpace);
+      }
 
       NextTokGetsSpace = false;
       continue;
Index: test/Preprocessor/macro_paste_commaext.c
===================================================================
--- test/Preprocessor/macro_paste_commaext.c
+++ test/Preprocessor/macro_paste_commaext.c
@@ -1,13 +1,20 @@
-// RUN: %clang_cc1 %s -E | grep 'V);'
-// RUN: %clang_cc1 %s -E | grep 'W, 1, 2);'
-// RUN: %clang_cc1 %s -E | grep 'X, 1, 2);'
-// RUN: %clang_cc1 %s -E | grep 'Y,);'
-// RUN: %clang_cc1 %s -E | grep 'Z,);'
+// RUN: %clang_cc1 %s -E | FileCheck --strict-whitespace --match-full-lines %s
+
+// In the following tests, note that the output is sensitive to the
+// whitespace *preceeding* the varargs argument, as well as to
+// interior whitespace. AFAIK, this is the only case where whitespace
+// preceeding an argument matters, and might be considered a bug in
+// GCC. Nevertheless, since this feature is a GCC extension in the
+// first place, we'll follow along.
 
 #define debug(format, ...) format, ## __VA_ARGS__)
+// CHECK:V);
 debug(V);
-debug(W, 1, 2);
+// CHECK:W,1, 2);
+debug(W,1, 2);
+// CHECK:X, 1, 2);
 debug(X, 1, 2 );
+// CHECK:Y,);
 debug(Y, );
+// CHECK:Z,);
 debug(Z,);
-
Index: test/Preprocessor/stringize_space.c
===================================================================
--- test/Preprocessor/stringize_space.c
+++ test/Preprocessor/stringize_space.c
@@ -18,3 +18,14 @@
     1)
 
 // CHECK: {{^}}"-1"
+
+#define paste(a,b) str(a<b##ld)
+paste(hello1, wor)
+paste(hello2,
+      wor)
+paste(hello3,
+wor)
+
+// CHECK: {{^}}"hello1<world"
+// CHECK: {{^}}"hello2<world"
+// CHECK: {{^}}"hello3<world"