c++: fix ICE in convert_nontype_argument [PR116384]

Here we ICE since r14-8291 in C++11/C++14 modes.  Fortunately
this is an easy one.

The important bit of r14-8291 is this:

@@ -20056,9 +20071,12 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
        RETURN (retval);
      }
    if (IMPLICIT_CONV_EXPR_NONTYPE_ARG (t))
-     /* We'll pass this to convert_nontype_argument again, we don't need
-        to actually perform any conversion here.  */
-     RETURN (expr);
+     {
+       tree r = convert_nontype_argument (type, expr, complain);
+       if (r == NULL_TREE)
+         r = error_mark_node;
+       RETURN (r);
+     }

which obviously means that instead of returning right away we go
to convert_nontype_argument.  When type is error_mark_node and we're
in C++17, in convert_nontype_argument we go down this path:

      else if (INTEGRAL_OR_ENUMERATION_TYPE_P (type)
               || cxx_dialect >= cxx17)
        {
          expr = build_converted_constant_expr (type, expr, complain);
          if (expr == error_mark_node)
            return (complain & tf_error) ? NULL_TREE : error_mark_node;
	  // ...
	}

but pre-C++17, we take a different route and end up crashing on
gcc_unreachable.

It would of course also work to check for error_mark_node early in
build_converted_constant_expr.

	PR c++/116384

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr) <case IMPLICIT_CONV_EXPR>: Bail if tsubst
	returns error_mark_node.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/vt-116384.C: New test.

2 files changed, 28 insertions, 0 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 8725a5e..684ee0c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -20217,6 +20217,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
     case IMPLICIT_CONV_EXPR:
       {
 	tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+	if (type == error_mark_node)
+	  RETURN (error_mark_node);
 	tree expr = RECUR (TREE_OPERAND (t, 0));
 	if (dependent_type_p (type) || type_dependent_expression_p (expr))
 	  {
diff --git a/gcc/testsuite/g++.dg/cpp0x/vt-116384.C b/gcc/testsuite/g++.dg/cpp0x/vt-116384.C
new file mode 100644
index 0000000..54d7f07
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/vt-116384.C
@@ -0,0 +1,26 @@
+// PR c++/116384
+// { dg-do compile { target c++11 } }
+
+namespace a {
+template <bool, typename> struct c;
+template <typename> struct d;
+}
+namespace e {
+namespace g {
+template <typename> using h = void;
+template <typename, template <typename> class, typename...> struct detector {};
+template <template <typename> class i, typename... args>
+struct detector<h<i<args...>>, i, args...>;
+}
+template <template <typename> class i, typename... args>
+using j = g::detector<void, i, args...>;
+template <bool b, typename k = void> using l = typename a::c<b, k>::m;
+template <typename> struct conjunction;
+namespace g {
+template <typename k> using n = l<conjunction<a::d<k>>::p>;
+}
+template <typename k, g::n<k> = true> class o;
+}
+struct r;
+template <typename k> using q = e::o<k>;
+void s() { e::j<q, r> f; }