c++: constexpr empty aggr [PR105795]

In this testcase, leaving ctx->ctor pointing to the enclosing object meant
that evaluating the initializer for the subobject clobbered previous
initializers for the enclosing object.  So do update ctx->ctor, just don't
add it to the enclosing object ctor.

	PR c++/105795

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_bare_aggregate): Always call
	init_subob_ctx.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-aggr-base1.C: New test.

2 files changed, 29 insertions, 5 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 4520847..021eaa3 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4777,12 +4777,9 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree t,
       tree orig_value = value;
       /* Like in cxx_eval_store_expression, omit entries for empty fields.  */
       bool no_slot = TREE_CODE (type) == RECORD_TYPE && is_empty_field (index);
-      if (no_slot)
-	new_ctx = *ctx;
-      else
-	init_subob_ctx (ctx, new_ctx, index, value);
+      init_subob_ctx (ctx, new_ctx, index, value);
       int pos_hint = -1;
-      if (new_ctx.ctor != ctx->ctor)
+      if (new_ctx.ctor != ctx->ctor && !no_slot)
 	{
 	  /* If we built a new CONSTRUCTOR, attach it now so that other
 	     initializers can refer to it.  */
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-aggr-base1.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-aggr-base1.C
new file mode 100644
index 0000000..06acb4a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-aggr-base1.C
@@ -0,0 +1,27 @@
+// PR c++/105795
+// { dg-do compile { target c++17 } }
+
+struct empty
+{};
+
+template <typename T>
+struct tuple_holder
+{
+    [[no_unique_address]] T value;
+};
+
+struct tuple : tuple_holder<int>, tuple_holder<empty>
+{};
+
+constexpr auto make_tuple(int&& i, empty&& e)
+{
+    return tuple{i, e};
+}
+
+constexpr int foo()
+{
+    auto tuple = make_tuple(1, empty{});
+    return static_cast<const tuple_holder<int>&>(tuple).value;
+}
+
+static_assert (foo() == 1);