[C++ PATCH] Fix a structured binding ICE (PR c++/83958)

[Jakub Jelinek <jakub at redhat dot com>]

Hi!

I've recently added the complete_type call in case the structured binding
is a reference to a type that needs to be instantiated.
This testcase shows a problem where it can't be instantiated and we ICE
because we don't expect an incomplete type.  If decl isn't a reference,
cp_finish_decl for it should handle everything.
I've considered using complete_type_or_else, but we aren't checking that
decl's type is complete, but rather that the type it refers to is complete,
and furthermore not sure if it is very nice to print <structured bindings>
in the diagnostics.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR c++/83958
	* decl.c (cp_finish_decomp): Diagnose if reference structure binding
	refers to incomplete type.

	* g++.dg/cpp1z/decomp35.C: New test.

--- gcc/cp/decl.c.jj	2018-01-19 23:34:36.634278658 +0100
+++ gcc/cp/decl.c	2018-01-22 12:16:39.316524652 +0100
@@ -7433,6 +7433,12 @@ cp_finish_decomp (tree decl, tree first,
       type = complete_type (TREE_TYPE (type));
       if (type == error_mark_node)
 	goto error_out;
+      if (!COMPLETE_TYPE_P (type))
+	{
+	  error_at (loc, "structured binding refers to incomplete type %qT",
+		    type);
+	  goto error_out;
+	}
     }
 
   tree eltype = NULL_TREE;
--- gcc/testsuite/g++.dg/cpp1z/decomp35.C.jj	2018-01-22 12:18:19.421399364 +0100
+++ gcc/testsuite/g++.dg/cpp1z/decomp35.C	2018-01-22 12:22:28.243046089 +0100
@@ -0,0 +1,35 @@
+// PR c++/83958
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+template <typename = void> struct A;
+class B;
+template <typename, typename, typename = A<>> class C;
+template <typename, typename> struct D;
+template <typename T, typename U, typename V, typename, typename, typename W>
+struct E {
+  using X = W;
+  X operator* ();
+  T operator++ ();
+  template <typename P, typename R, typename S, typename Q>
+  bool operator!= (E<P, U, V, R, S, Q>);
+};
+template <typename T, typename U, typename>
+struct F {
+  class G;
+  using H = D<T, U>;
+  using I = E<G, T, U, G, H, H &>;
+  class G : public I {};
+  G begin ();
+  G end ();
+};
+template <typename T, typename U, typename V> struct C : F<T, U, V> {
+  using J = F<T, U, V>;
+  using J::begin;
+  using J::end;
+};
+using K = class L;
+struct M {
+  void foo () { for (auto & [ a ] : m) {} }	// { dg-error "incomplete type" }
+  C<K, B> m;					// { dg-warning "only available with" "" { target c++14_down } .-1 }
+};

	Jakub