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)
@@ -1576,14 +1576,17 @@
 
 
 /* Returns true if a macro has been defined.
+   If no_underscore is true, check that the macro
+   name does not start with underscore.
    This might not work if compile with -save-temps,
    or preprocess separately from compilation.  */
 
 static bool
-is_macro(cpp_reader *pfile, const uchar *base)
+is_macro(cpp_reader *pfile, const uchar *base, bool no_underscore)
 {
   const uchar *cur = base;
-  if (! ISIDST (*cur))
+  bool invalid_ident = (no_underscore ? ! ISALPHA (*cur) : ! ISIDST (*cur));
+  if (invalid_ident)
     return false;
   unsigned int hash = HT_HASHSTEP (0, *cur);
   ++cur;
@@ -1872,7 +1875,7 @@
 	 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))
+      if (is_macro (pfile, cur, true))
 	{
 	  /* Raise a warning, but do not consume subsequent tokens.  */
 	  if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)
@@ -2002,7 +2005,7 @@
 	 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))
+      if (is_macro (pfile, cur, true))
 	{
 	  /* Raise a warning, but do not consume subsequent tokens.  */
 	  if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)
@@ -2023,7 +2026,7 @@
 	}
     }
   else if (CPP_OPTION (pfile, cpp_warn_cxx11_compat)
-	   && is_macro (pfile, cur)
+	   && is_macro (pfile, cur, false)
 	   && !pfile->state.skipping)
     cpp_warning_with_line (pfile, CPP_W_CXX11_COMPAT,
 			   token->src_loc, 0, "C++11 requires a space "