c++: Handle RANGE_EXPR index in init_subob_ctx [PR97328]

In the testcase below, we're ICEing during constexpr evaluation of the
CONSTRUCTOR {.data={{}, [1 ... 7]={}}} of type 'vector'.  The interesting
thing about this CONSTRUCTOR is that it has a RANGE_EXPR index for an
element initializer which doesn't satisfy reduced_constant_expression_p
(because the field 't' is uninitialized).

This is a problem because init_subob_ctx currently punts on setting up a
sub-aggregate initialization context when given a RANGE_EXPR index, so
we later trip over the asserts in verify_ctor_sanity when recursing into
cxx_eval_bare_aggregate on this element initializer.

Fix this by making init_subob_ctx set up an appropriate initialization
context when supplied a RANGE_EXPR index.

gcc/cp/ChangeLog:

	PR c++/97328
	* constexpr.c (init_subob_ctx): Don't punt on RANGE_EXPR
	indexes, instead build a sub-aggregate initialization context
	with no subobject.

gcc/testsuite/ChangeLog:

	PR c++/97328
	* g++.dg/cpp2a/constexpr-init19.C: New test.
	* g++.dg/cpp2a/constexpr-init20.C: New test.

gcc/cp/constexpr.c

@@ -3954,7 +3954,8 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   new_ctx = *ctx;
 
   if (index && TREE_CODE (index) != INTEGER_CST
-      && TREE_CODE (index) != FIELD_DECL)
+      && TREE_CODE (index) != FIELD_DECL
+      && TREE_CODE (index) != RANGE_EXPR)
     /* This won't have an element in the new CONSTRUCTOR.  */
     return;
 
@@ -3967,7 +3968,13 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
      update object to refer to the subobject and ctor to refer to
      the (newly created) sub-initializer.  */
   if (ctx->object)
-    new_ctx.object = build_ctor_subob_ref (index, type, ctx->object);
+    {
+      if (index == NULL_TREE || TREE_CODE (index) == RANGE_EXPR)
+	/* There's no well-defined subobject for this index.  */
+	new_ctx.object = NULL_TREE;
+      else
+	new_ctx.object = build_ctor_subob_ref (index, type, ctx->object);
+    }
   tree elt = build_constructor (type, NULL);
   CONSTRUCTOR_NO_CLEARING (elt) = true;
   new_ctx.ctor = elt;

gcc/testsuite/g++.dg/cpp2a/constexpr-init19.C

@@ -0,0 +1,15 @@
+// PR c++/97328
+// { dg-do compile { target c++20 } }
+
+struct vector {
+  struct storage {
+    int t;
+    constexpr storage() {}
+  } data[8];
+};
+
+constexpr auto foo() {
+  vector i;
+  return i;
+}
+auto val = foo();

gcc/testsuite/g++.dg/cpp2a/constexpr-init20.C

@@ -0,0 +1,15 @@
+// PR c++/97328
+// { dg-do compile { target c++20 } }
+
+struct vector {
+  union storage {
+    int t;
+    constexpr storage() {}
+  } data[8];
+};
+
+constexpr auto foo() {
+  vector i;
+  return i;
+}
+auto val = foo();