Index: lib/Lex/TokenLexer.cpp
===================================================================
--- lib/Lex/TokenLexer.cpp	(revision 234596)
+++ lib/Lex/TokenLexer.cpp	(working copy)
@@ -521,6 +521,13 @@
 /// are more ## after it, chomp them iteratively.  Return the result as Tok.
 /// If this returns true, the caller should immediately return the token.
 bool TokenLexer::PasteTokens(Token &Tok) {
+  // MSVC: If previous token was pasted, this must be a recovery from an invalid
+  // paste operation. Ignore spaces before this token to mimic MSVC output.
+  // Required for generating valid UUID strings in some MS headers.
+  if (PP.getLangOpts().MicrosoftExt && (CurToken >= 2) &&
+      Tokens[CurToken - 2].is(tok::hashhash))
+    Tok.clearFlag(Token::LeadingSpace);
+  
   SmallString<128> Buffer;
   const char *ResultTokStrPtr = nullptr;
   SourceLocation StartLoc = Tok.getLocation();
Index: test/Preprocessor/macro_paste_msextensions.c
===================================================================
--- test/Preprocessor/macro_paste_msextensions.c	(revision 234596)
+++ test/Preprocessor/macro_paste_msextensions.c	(working copy)
@@ -32,3 +32,10 @@
 bar(q)
 
 // CHECK: abc(baz(q))
+
+
+#define str(x) #x
+#define collapse_spaces(a, b, c, d) str(a ## - ## b ## - ## c ## d)
+collapse_spaces(1a, b2, 3c, d4)
+
+// CHECK: "1a-b2-3cd4"