c++: constexpr array reference and value-initialization [PR101371]

This PR gave me a hard time: I saw multiple issues starting with
different revisions.  But ultimately the root cause seems to be
the following, and the attached patch fixes all issues I've found
here.

In cxx_eval_array_reference we create a new constexpr context for the
CP_AGGREGATE_TYPE_P case, but we also have to create it for the
non-aggregate case.  In this test, we are evaluating

  ((B *)this)->a = rhs->a

which means that we set ctx.object to ((B *)this)->a.  Then we proceed
to evaluate the initializer, rhs->a.  For *rhs, we eval rhs, a PARM_DECL,
for which we have (const B &) &c.arr[0] in the hash table.  Then
cxx_fold_indirect_ref gives us c.arr[0].  c is evaluated to {.arr={}} so
c.arr is {}.  Now we want c.arr[0], so we end up in cxx_eval_array_reference
and since we're initializing from {}, we call build_value_init which
gives us an AGGR_INIT_EXPR that calls 'constexpr B::B()'.  Then we
evaluate this AGGR_INIT_EXPR and since its first argument is dummy,
we take ctx.object instead.  But that is the wrong object, we're not
initializing ((B *)this)->a here.  And so we wound up with an
initializer for A, and then crash in cxx_eval_component_reference:

  gcc_assert (DECL_CONTEXT (part) == TYPE_MAIN_VARIANT (TREE_TYPE (whole)));

where DECL_CONTEXT (part) is B (as it should be) but the type of whole
was A.

So create a new object, if there already was one, and the element type
is not a scalar.

	PR c++/101371

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_array_reference): Create a new .object
	and .ctor for the non-aggregate non-scalar case too when
	value-initializing.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-101371-2.C: New test.
	* g++.dg/cpp1y/constexpr-101371.C: New test.

1 files changed, 11 insertions, 4 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 39787f3..31fa5b6 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3851,16 +3851,23 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
     {
       tree empty_ctor = build_constructor (init_list_type_node, NULL);
       val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
+    }
+  else
+    val = build_value_init (elem_type, tf_warning_or_error);
+
+  if (!SCALAR_TYPE_P (elem_type))
+    {
       new_ctx = *ctx;
-      new_ctx.object = t;
+      if (ctx->object)
+	/* If there was no object, don't add one: it could confuse us
+	   into thinking we're modifying a const object.  */
+	new_ctx.object = t;
       new_ctx.ctor = build_constructor (elem_type, NULL);
       ctx = &new_ctx;
     }
-  else
-    val = build_value_init (elem_type, tf_warning_or_error);
   t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
 				    overflow_p);
-  if (CP_AGGREGATE_TYPE_P (elem_type) && t != ctx->ctor)
+  if (!SCALAR_TYPE_P (elem_type) && t != ctx->ctor)
     free_constructor (ctx->ctor);
   return t;
 }