Index: gcc/testsuite/g++.dg/cpp0x/udlit-macros.C
===================================================================
--- gcc/testsuite/g++.dg/cpp0x/udlit-macros.C	(revision 0)
+++ gcc/testsuite/g++.dg/cpp0x/udlit-macros.C	(working copy)
@@ -0,0 +1,7 @@
+// PR c++/80955
+// { dg-do compile { target c++11 } }
+
+using size_t = decltype(sizeof(0));
+#define _zero
+int operator""_zero(const char*, size_t) { return 0; }
+int main() { return ""_zero; }
Index: libcpp/lex.c
===================================================================
--- libcpp/lex.c	(revision 253775)
+++ libcpp/lex.c	(working copy)
@@ -2001,8 +2001,11 @@
       /* If a string format macro, say from inttypes.h, is placed touching
 	 a string literal it could be parsed as a C++11 user-defined string
 	 literal thus breaking the program.
-	 Try to identify macros with is_macro. A warning is issued. */
-      if (is_macro (pfile, cur))
+	 Try to identify macros with is_macro. A warning is issued.
+	 Don't do this for a user-defined literal, i.e. an
+	 empty string followed by an identifier.
+	 For an empty string "", (cur-base)==2. Bug 80955 */
+      if (is_macro (pfile, cur) && ((cur-base) != 2))
 	{
 	  /* Raise a warning, but do not consume subsequent tokens.  */
 	  if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)