C++ PATCH for c++/55753 (ICE with constexpr)

[Jason Merrill]

This is a bit I missed in my 55419 patch; if we aren't setting 
TREE_CONSTANT on TARGET_EXPRs in the constexpr code, we can't expect it 
in the template code.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.7.
commit db160eb9b07b62f3696e7358c74e6d59c68385d8
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Jan 3 14:43:05 2013 -0500
    	PR c++/55419
    	PR c++/55753
    	* pt.c (tsubst_copy_and_build) [TARGET_EXPR]: Don't touch
    	TREE_CONSTANT.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1b3f039..09a0aa5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -14333,11 +14333,9 @@ tsubst_copy_and_build (tree t,
     case TARGET_EXPR:
       /* We can get here for a constant initializer of non-dependent type.
          FIXME stop folding in cp_parser_initializer_clause.  */
-      gcc_assert (TREE_CONSTANT (t));
       {
 	tree r = get_target_expr_sfinae (RECUR (TARGET_EXPR_INITIAL (t)),
 					 complain);
-	TREE_CONSTANT (r) = true;
 	RETURN (r);
       }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor12.C
new file mode 100644
index 0000000..a5a4b4d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor12.C
@@ -0,0 +1,14 @@
+// PR c++/55753
+// { dg-options -std=c++11 }
+
+template <typename Tp>
+struct C {
+  constexpr C(const Tp& r) { }
+};
+
+template <typename Tp>
+struct B {
+  B() {
+    C<double> cpl = C<double>((true ? 1.0 : C<double>()));
+  }
+};