c++: initialized array of vla [PR58646]

We went into build_vec_init because we're dealing with a VLA, but then
build_vec_init thought it was safe to just build an INIT_EXPR because the
outer dimension is constant.  Nope.

	PR c++/58646

gcc/cp/ChangeLog:

	* init.cc (build_vec_init): Check for vla element type.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/vla24.C: New test.

gcc/cp/init.cc

@@ -4395,6 +4395,7 @@ build_vec_init (tree base, tree maxindex, tree init,
   if (init
       && TREE_CODE (atype) == ARRAY_TYPE
       && TREE_CONSTANT (maxindex)
+      && !vla_type_p (type)
       && (from_array == 2
 	  ? vec_copy_assign_is_trivial (inner_elt_type, init)
 	  : !TYPE_NEEDS_CONSTRUCTING (type))

gcc/testsuite/g++.dg/ext/vla24.C

@@ -0,0 +1,7 @@
+// PR c++/58646
+// { dg-additional-options -Wno-vla }
+
+void foo(int n)
+{
+  int a[2][n] = {};
+}