c++: Fix verify_ctor_sanity ICE [PR96241]

The code added in r10-6437 caused us to create a CONSTRUCTOR when we're
{}-initializing an aggregate.  Then we pass this new CONSTRUCTOR down to
cxx_eval_constant_expression which, if the CONSTRUCTOR isn't TREE_CONSTANT
or reduced_constant_expression_p, calls cxx_eval_bare_aggregate.  In
this case the CONSTRUCTOR wasn't reduced_constant_expression_p because
for r_c_e_p a CONST_DECL isn't good enough so it returns false.  So we
go to cxx_eval_bare_aggregate where we crash, because ctx->ctor wasn't
set up properly.  So my fix is to do so.  Since we're value-initializing,
I'm not setting CONSTRUCTOR_NO_CLEARING.  To avoid keeping a garbage
constructor around, I call free_constructor in case the evaluation did
not use it.

gcc/cp/ChangeLog:

	PR c++/96241
	* constexpr.c (cxx_eval_array_reference): Set up ctx->ctor if we
	are initializing an aggregate.  Call free_constructor on the new
	CONSTRUCTOR if it isn't returned from cxx_eval_constant_expression.

gcc/testsuite/ChangeLog:

	PR c++/96241
	* g++.dg/cpp0x/constexpr-96241.C: New test.
	* g++.dg/cpp1y/constexpr-96241.C: New test.

gcc/cp/constexpr.c

@@ -3657,15 +3657,22 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
      initializer, it's initialized from {}.  But use build_value_init
      directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
   tree val;
+  constexpr_ctx new_ctx;
   if (CP_AGGREGATE_TYPE_P (elem_type))
     {
       tree empty_ctor = build_constructor (init_list_type_node, NULL);
       val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
+      new_ctx = *ctx;
+      new_ctx.ctor = build_constructor (elem_type, NULL);
+      ctx = &new_ctx;
     }
   else
     val = build_value_init (elem_type, tf_warning_or_error);
-  return cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
-				       overflow_p);
+  t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
+				    overflow_p);
+  if (CP_AGGREGATE_TYPE_P (elem_type) && t != ctx->ctor)
+    free_constructor (ctx->ctor);
+  return t;
 }
 
 /* Subroutine of cxx_eval_constant_expression.

gcc/testsuite/g++.dg/cpp0x/constexpr-96241.C

@@ -0,0 +1,18 @@
+// PR c++/96241
+// { dg-do compile { target c++11 } }
+
+template <typename T, T...> struct S {};
+template <typename T, T t> using U = S<T, __integer_pack(t)...>;
+template <long... N> using f = S<unsigned long, N...>;
+template <long N> using V = U<unsigned long, N>;
+template <int N> struct A { typedef int type[N]; };
+template <int N> struct B { typename A<N>::type k; };
+template <typename T, int N, unsigned long... P>
+constexpr B<N> bar(T (&arr)[N], f<P...>) {
+  return {arr[P]...};
+}
+template <typename T, int N> constexpr B<N> foo(T (&arr)[N]) {
+  return bar(arr, V<N>{});
+}
+constexpr char arr[2]{};
+B<2> b = foo(arr);

gcc/testsuite/g++.dg/cpp1y/constexpr-96241.C

@@ -0,0 +1,47 @@
+// PR c++/96241
+// { dg-do compile { target c++14 } }
+
+#define assert(expr) static_assert (expr, #expr)
+
+enum E { o };
+
+struct S {
+  int e = o;
+};
+
+using T = S[3];
+
+constexpr struct S s[1][1][1] = { };
+assert (0 == s[0][0][0].e);
+
+constexpr int
+fn0 ()
+{
+  return T{}[0].e;
+}
+assert(fn0 () == 0);
+
+constexpr int
+fn1 ()
+{
+  S d[1];
+  int x = d[0].e;
+  return x;
+}
+assert(fn1 () == 0);
+
+constexpr int
+fn2 ()
+{
+  S d[1];
+  return d[0].e;
+}
+assert(fn2 () == 0);
+
+constexpr int
+fn3 ()
+{
+  struct X { int e = o; } d[1]{};
+  return d[0].e;
+}
+assert(fn3 () == 0);