[C++ PATCH] Fix ICE in C++14 with null RETURN_EXPR (PR c++/65202)

[Marek Polacek]

We ICE on this invalid testcase in C++14 because in C++14 a function returning
void is a valid constexpr function, so adl_swap is registered as one, while in
C++11 it is not registered.  Then later on, we weren't able to properly handle
a RETURN_EXPR with null operand when trying to evaluate the expression.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2015-02-26  Marek Polacek  <pola...@redhat.com>

        PR c++/65202
        * constexpr.c (cxx_eval_constant_expression): Handle null tree.

        * g++.dg/cpp1y/pr65202.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 32a23ff7..1d2bb2f 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -2935,7 +2935,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, 
tree t,
   constexpr_ctx new_ctx;
   tree r = t;
 
-  if (t == error_mark_node)
+  if (t == NULL_TREE || t == error_mark_node)
     {
       *non_constant_p = true;
       return t;
diff --git gcc/testsuite/g++.dg/cpp1y/pr65202.C 
gcc/testsuite/g++.dg/cpp1y/pr65202.C
index e69de29..602b264 100644
--- gcc/testsuite/g++.dg/cpp1y/pr65202.C
+++ gcc/testsuite/g++.dg/cpp1y/pr65202.C
@@ -0,0 +1,26 @@
+// // PR c++/65202
+// { dg-do compile { target c++14 } }
+
+template <typename T> struct is_move_constructible;
+template <typename T> struct is_move_assignable;
+template <int, typename T> using enable_if_t = int;
+namespace adl {
+template <
+    typename L, typename R,
+    enable_if_t<is_move_constructible<L>() && is_move_assignable<L>(), int>...>
+constexpr auto adl_swap(L &l, R &r) -> decltype(swap(l, r)) {
+  return;
+}
+template <typename L, typename R>
+auto swap(L &l, R &r) noexcept(noexcept(adl::adl_swap(l, r)))
+    -> decltype(adl::adl_swap(l, r));
+namespace ns {
+template <typename T> struct foo {};
+template <typename T> void swap(foo<T> &, foo<T> &);
+struct bar;
+
+int main()
+{
+    foo<ns::bar> f;
+    adl::swap(f, f)
+} // { dg-error "" }

        Marek