C++ PATCH for c++/55720 (link error with lambda conversion)

[Jason Merrill]

Most of the G++ linkage code really needs to be torn out in favor of 
just using cgraph.  Until that happens, we need to set TREE_USED here so 
that decl_needed_p will return true and allow cgraph to consider the 
function.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.7.
commit 701e03d86ffb5c36e1c907e84b8b0dc504ebe5b2
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Feb 13 12:52:51 2013 -0500
    	PR c++/55720
    	* semantics.c (maybe_add_lambda_conv_op): Mark static thunk
    	TREE_USED.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 46c2e64..95158a5 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -9559,6 +9559,8 @@ maybe_add_lambda_conv_op (tree type)
   body = begin_function_body ();
   compound_stmt = begin_compound_stmt (0);
 
+  /* decl_needed_p needs to see that it's used.  */
+  TREE_USED (statfn) = 1;
   finish_return_stmt (decay_conversion (statfn, tf_warning_or_error));
 
   finish_compound_stmt (compound_stmt);
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv7.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv7.C
new file mode 100644
index 0000000..861c91c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv7.C
@@ -0,0 +1,20 @@
+// PR c++/55720
+// { dg-do link { target c++11 } }
+
+template <class T>
+struct X {
+  static void (*code) ();
+};
+
+template <class T>
+void (*X<T>::code) () = []{};  // Line 7
+
+struct Y {
+  void (*code) () = []{} ; // Line 10
+  void operator()() { code(); }
+};
+
+int main () {
+  X<int>::code();
+  Y()();
+}