C++ PATCH for c++/51526 (constexpr delegating constructor)

[Jason Merrill]

For a constexpr delegating constructor, we should be able to just use 
the delegated initializer.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 4443d34fb34484de797d087066dfe87fb55afc1f
Author: Jason Merrill <ja...@redhat.com>
Date:   Mon Dec 19 15:50:09 2011 -0500
    	PR c++/51526
    	* semantics.c (build_data_member_initialization): Handle
    	delegating constructor.
    	(build_constexpr_constructor_member_initializers): Likewise.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 2788daa..ec56ab5 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -5801,6 +5801,12 @@ build_data_member_initialization (tree t, VEC(constructor_elt,gc) **vec)
 	     the const_cast.  */
 	  member = op;
 	}
+      else if (op == current_class_ptr
+	       && (same_type_ignoring_top_level_qualifiers_p
+		   (TREE_TYPE (TREE_TYPE (member)),
+		    current_class_type)))
+	/* Delegating constructor.  */
+	member = op;
       else
 	{
 	  /* We don't put out anything for an empty base.  */
@@ -5907,7 +5913,20 @@ build_constexpr_constructor_member_initializers (tree type, tree body)
   else
     gcc_assert (errorcount > 0);
   if (ok)
-    return build_constructor (type, vec);
+    {
+      if (VEC_length (constructor_elt, vec) > 0)
+	{
+	  /* In a delegating constructor, return the target.  */
+	  constructor_elt *ce = VEC_index (constructor_elt, vec, 0);
+	  if (ce->index == current_class_ptr)
+	    {
+	      body = ce->value;
+	      VEC_free (constructor_elt, gc, vec);
+	      return body;
+	    }
+	}
+      return build_constructor (type, vec);
+    }
   else
     return error_mark_node;
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-delegating.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-delegating.C
new file mode 100644
index 0000000..a020888
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-delegating.C
@@ -0,0 +1,15 @@
+// PR c++/51526
+// { dg-options -std=c++0x }
+
+const int j = 42;
+
+struct S {
+  int i;
+  constexpr S(int i) : i(i) {}
+  constexpr S() : S(j) {}
+};
+
+constexpr S s{};
+
+#define SA(X) static_assert((X),#X)
+SA(s.i == 42);