C++ PATCH for c++/48089 (ICE with invalid constexpr ctor)

[Jason Merrill]

In a normal constexpr function, we treat *this as a potential constant 
expression.  But in a constexpr constructor it isn't, since we're still 
in the process of initializing the object.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 0f23f8a6fca94b1a00a51ea34643268e9ff840e9
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Mar 16 20:55:52 2011 -0400
        PR c++/48089
        * semantics.c (potential_constant_expression_1): Don't allow *this
        in a constructor.
        (register_constexpr_fundef): Use potential_rvalue_constant_expression.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index a0c5ae3..7519d26 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -5674,11 +5674,11 @@ register_constexpr_fundef (tree fun, tree body)
       body = unshare_expr (TREE_OPERAND (body, 0));
     }
 
-  if (!potential_constant_expression (body))
+  if (!potential_rvalue_constant_expression (body))
     {
       DECL_DECLARED_CONSTEXPR_P (fun) = false;
       if (!DECL_TEMPLATE_INSTANTIATION (fun))
-       require_potential_constant_expression (body);
+       require_potential_rvalue_constant_expression (body);
       return NULL;
     }
   fundef->body = body;
@@ -7496,7 +7496,16 @@ potential_constant_expression_1 (tree t, bool want_rval, 
tsubst_flags_t flags)
         tree x = TREE_OPERAND (t, 0);
         STRIP_NOPS (x);
         if (is_this_parameter (x))
-         return true;
+         {
+           if (DECL_CONSTRUCTOR_P (DECL_CONTEXT (x)) && want_rval)
+             {
+               if (flags & tf_error)
+                 error ("the value of the object being constructed is "
+                        "not a constant expression");
+               return false;
+             }
+           return true;
+         }
        return potential_constant_expression_1 (x, rval, flags);
       }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-48089.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-48089.C
new file mode 100644
index 0000000..88ef3d6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-48089.C
@@ -0,0 +1,9 @@
+// PR c++/48089
+// { dg-options -std=c++0x }
+
+struct s {
+    constexpr s() : v(v) { }   // { dg-error "object being constructed" }
+    char v;
+};
+
+s bang;