C++ PATCH for c++/85285, ICE with flexible array after substitution

[Jason Merrill]

Checking for a flexible array in a union is currently done by
grokdeclarator, so it wasn't being caught after substitution.  I
didn't see a good place to handle it for both cases, and it's small
enough, so I just copied it over.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit e63bd3e7471152b3c39cbfc58e735e3efeb458b7
Author: Jason Merrill <ja...@redhat.com>
Date:   Mon Apr 9 17:24:19 2018 -0400

            PR c++/85285 - ICE with flexible array after substitution.
    
            * pt.c (instantiate_class_template_1): Check for flexible array in
            union.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index db3d7e38d85..76e546cdeaa 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10904,6 +10904,14 @@ instantiate_class_template_1 (tree type)
 			      cxx_incomplete_type_error (r, rtype);
 			      TREE_TYPE (r) = error_mark_node;
 			    }
+			  else if (TREE_CODE (rtype) == ARRAY_TYPE
+				   && TYPE_DOMAIN (rtype) == NULL_TREE
+				   && (TREE_CODE (type) == UNION_TYPE
+				       || TREE_CODE (type) == QUAL_UNION_TYPE))
+			    {
+			      error ("flexible array member %qD in union", r);
+			      TREE_TYPE (r) = error_mark_node;
+			    }
 			}
 
 		      /* If it is a TYPE_DECL for a class-scoped ENUMERAL_TYPE,
diff --git a/gcc/testsuite/g++.dg/ext/flexary30.C b/gcc/testsuite/g++.dg/ext/flexary30.C
new file mode 100644
index 00000000000..59b7587594c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/flexary30.C
@@ -0,0 +1,8 @@
+// PR c++/85285
+
+template<typename T> union A
+{
+  T x;				// { dg-error "flexible array" }
+};
+
+A<int[]> a;