C++ PATCH for c++/69842 (wrong error with generic lambda)

[Jason Merrill]

The problem here was that the call from the stub returned by the 
conversion function to the op() was changing an xvalue to an lvalue, 
leading to a parameter of the wrong type in the op().

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 54448906d76dd9df2523e6c3950ed368cb63b4de
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Feb 17 15:20:02 2016 -0500
    	PR c++/69842
    	* method.c (forward_parm): Split out from...
    	(add_one_base_init): ...here.
    	* lambda.c (maybe_add_lambda_conv_op): Use it.

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 3b91089..7800ae8 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6013,6 +6013,7 @@ extern tree make_thunk				(tree, bool, tree, tree);
 extern void finish_thunk			(tree);
 extern void use_thunk				(tree, bool);
 extern bool trivial_fn_p			(tree);
+extern tree forward_parm			(tree);
 extern bool is_trivially_xible			(enum tree_code, tree, tree);
 extern tree get_defaulted_eh_spec		(tree);
 extern tree unevaluated_noexcept_spec		(void);
diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
index 93b192c..296c6f7 100644
--- a/gcc/cp/lambda.c
+++ b/gcc/cp/lambda.c
@@ -962,7 +962,9 @@ maybe_add_lambda_conv_op (tree type)
 	      }
 	    else
 	      {
-		tree a = convert_from_reference (tgt);
+		++processing_template_decl;
+		tree a = forward_parm (tgt);
+		--processing_template_decl;
 		CALL_EXPR_ARG (call, ix) = a;
 		if (decltype_call)
 		  CALL_EXPR_ARG (decltype_call, ix) = copy_node (a);
diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index e358ebd..f455b32 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -474,6 +474,19 @@ trivial_fn_p (tree fn)
   return type_has_trivial_fn (DECL_CONTEXT (fn), special_function_p (fn));
 }
 
+/* PARM is a PARM_DECL for a function which we want to forward to another
+   function without changing its value category, a la std::forward.  */
+
+tree
+forward_parm (tree parm)
+{
+  tree exp = convert_from_reference (parm);
+  if (TREE_CODE (TREE_TYPE (parm)) != REFERENCE_TYPE
+      || TYPE_REF_IS_RVALUE (TREE_TYPE (parm)))
+    exp = move (exp);
+  return exp;
+}
+
 /* Subroutine of do_build_copy_constructor: Add a mem-initializer for BINFO
    given the parameter or parameters PARM, possibly inherited constructor
    base INH, or move flag MOVE_P.  */
@@ -494,10 +507,7 @@ add_one_base_init (tree binfo, tree parm, bool move_p, tree inh,
       init = NULL_TREE;
       for (; parm; parm = DECL_CHAIN (parm))
 	{
-	  tree exp = convert_from_reference (parm);
-	  if (TREE_CODE (TREE_TYPE (parm)) != REFERENCE_TYPE
-	      || TYPE_REF_IS_RVALUE (TREE_TYPE (parm)))
-	    exp = move (exp);
+	  tree exp = forward_parm (parm);
 	  *p = build_tree_list (NULL_TREE, exp);
 	  p = &TREE_CHAIN (*p);
 	}
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv1.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv1.C
new file mode 100644
index 0000000..6569af4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv1.C
@@ -0,0 +1,14 @@
+// PR c++/69842
+// { dg-do compile { target c++14 } }
+
+template <class T, class U> struct same;
+template <class T> struct same<T,T> {};
+
+int main()
+{
+  auto g = [](auto && _var) {
+    same<int&&,decltype(_var)>();
+  };
+
+  g(0);
+}