c++: fix ICE with constexpr ARRAY_REF [PR110382]

This code in cxx_eval_array_reference has been hard to get right.
In r12-2304 I added some code; in r13-5693 I removed some of it.

Here the problematic line is "S s = arr[0];" which causes a crash
on the assert in verify_ctor_sanity:

  gcc_assert (!ctx->object || !DECL_P (ctx->object)
              || ctx->global->get_value (ctx->object) == ctx->ctor);

ctx->object is the VAR_DECL 's', which is correct here.  The second
line points to the problem: we replaced ctx->ctor in
cxx_eval_array_reference:

  new_ctx.ctor = build_constructor (elem_type, NULL); // #1

which I think we shouldn't have; the CONSTRUCTOR we created in
cxx_eval_constant_expression/DECL_EXPR

  new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);

had the right type.

We still need #1 though.  E.g., in constexpr-96241.C, we never
set ctx.ctor/object before calling cxx_eval_array_reference, so
we have to build a CONSTRUCTOR there.  And in constexpr-101371-2.C
we have a ctx.ctor, but it has the wrong type, so we need a new one.

We can fix the problem by always clearing the object, and, as an
optimization, only create/free a new ctor when actually needed.

	PR c++/110382

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Create a new constructor
	only when we don't already have a matching one.  Clear the object
	when the type is non-scalar.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-110382.C: New test.

gcc/cp/constexpr.cc

@@ -4291,15 +4291,24 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
   else
     val = build_value_init (elem_type, tf_warning_or_error);
 
-  if (!SCALAR_TYPE_P (elem_type))
+  /* Create a new constructor only if we don't already have a suitable one.  */
+  const bool new_ctor = (!SCALAR_TYPE_P (elem_type)
+			 && (!ctx->ctor
+			     || !same_type_ignoring_top_level_qualifiers_p
+				  (elem_type, TREE_TYPE (ctx->ctor))));
+  if (new_ctor)
     {
       new_ctx = *ctx;
+      /* We clear the object here.  We used to replace it with T, but that
+	 caused problems (101371, 108158); and anyway, T is the initializer,
+	 not the target object.  */
+      new_ctx.object = NULL_TREE;
       new_ctx.ctor = build_constructor (elem_type, NULL);
       ctx = &new_ctx;
     }
   t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
 				    overflow_p);
-  if (!SCALAR_TYPE_P (elem_type) && t != ctx->ctor)
+  if (new_ctor && t != ctx->ctor)
     free_constructor (ctx->ctor);
   return t;
 }

gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C

@@ -0,0 +1,17 @@
+// PR c++/110382
+// { dg-do compile { target c++14 } }
+
+struct S {
+  double a = 0;
+};
+
+constexpr double
+g ()
+{
+  S arr[1];
+  S s = arr[0];
+  (void) arr[0];
+  return s.a;
+}
+
+int main() { return  g (); }