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 +1,31 @@
+// PR c++/80955
+// { dg-do compile { target c++11 } }
+
+#define __STDC_FORMAT_MACROS
+#include <inttypes.h>
+#include <stdio.h>
+#include <string.h>
+
+using size_t = decltype(sizeof(0));
+#define _zero
+#define _ID _xx
+int operator""_zero(const char*, size_t) { return 0; }
+int operator""_ID(const char*, size_t) { return 0; }
+
+int main()
+{
+  int64_t i64 = 123;
+  char buf[100];
+  sprintf(buf, "%"PRId64"abc", i64);  // { dg-warning "invalid suffix on literal" }
+  return strcmp(buf, "123abc")
+	 + ""_zero
+	 + "bob"_zero
+	 + R"#(raw
+	       string)#"_zero
+	 + "xx"_ID
+	 + ""_ID
+	 + R"AA(another
+		raw
+		string)AA"_ID;
+}
+
Index: libcpp/lex.c
===================================================================
--- libcpp/lex.c	(revision 254048)
+++ libcpp/lex.c	(working copy)
@@ -1871,8 +1871,9 @@
       /* 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.
+	 The macro name should not start with '_' for this warning. */
+      if ((*cur != '_') && is_macro (pfile, cur))
 	{
 	  /* Raise a warning, but do not consume subsequent tokens.  */
 	  if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)
@@ -2001,8 +2002,9 @@
       /* 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.
+	 The macro name should not start with '_' for this warning. */
+      if ((*cur != '_') && is_macro (pfile, cur))
 	{
 	  /* Raise a warning, but do not consume subsequent tokens.  */
 	  if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)