re PR c++/56481 (endless loop compiling a C++ file)

	PR c++/56481
	* semantics.c (potential_constant_expression_1): Use of 'this' in
	a non-constexpr function makes the expression not potentially
	constant.

From-SVN: r196737

gcc/cp/ChangeLog

 2013-03-16  Jason Merrill  <jason@redhat.com>
 
+	PR c++/56481
+	* semantics.c (potential_constant_expression_1): Use of 'this' in
+	a non-constexpr function makes the expression not potentially
+	constant.
+
 	N3276
 	PR c++/52748
 	* cp-tree.h (tsubst_flags): Add tf_decltype.

gcc/cp/semantics.c

@@ -8497,6 +8497,13 @@ potential_constant_expression_1 (tree t, bool want_rval, tsubst_flags_t flags)
         STRIP_NOPS (x);
         if (is_this_parameter (x))
 	  {
+	    if (DECL_CONTEXT (x)
+		&& !DECL_DECLARED_CONSTEXPR_P (DECL_CONTEXT (x)))
+	      {
+		if (flags & tf_error)
+		  error ("use of %<this%> in a constant expression");
+		return false;
+	      }
 	    if (want_rval && DECL_CONTEXT (x)
 		&& DECL_CONSTRUCTOR_P (DECL_CONTEXT (x)))
 	      {