c++: fix constexpr union with empty member [PR123346]

r14-2820 changed cxx_eval_bare_aggregate to set no_slot based on whether
new_ctx.ctor is NULL_TREE, to handle empty subobject elision. However
this incorrectly omits entries for empty union members, which later need
the entry to exist.
This caused valid code to be rejected as non-constant after gcc 13.3,
and in trunk caused an ICE when the diagnostic code tries to print a
CONSTRUCTOR with a null value.

	PR c++/123346

gcc/cp/ChangeLog:

	* constexpr.cc (init_subob_ctx): Do initialize new_ctx.ctor
	for an empty union member.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-union10.C: New test.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@gmail.com>
Co-authored-by: Jason Merrill <jason@redhat.com>

Author: riogu

gcc/cp/constexpr.cc

@@ -6692,8 +6692,17 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
   if (!AGGREGATE_TYPE_P (type) && !VECTOR_TYPE_P (type))
     /* A non-aggregate member doesn't get its own CONSTRUCTOR.  */
     return;
+
+  tree ctxtype = NULL_TREE;
+  if (ctx->ctor)
+    ctxtype = TREE_TYPE (ctx->ctor);
+  else if (ctx->object)
+    ctxtype = TREE_TYPE (ctx->object);
+  else
+    gcc_unreachable ();
+
   if (VECTOR_TYPE_P (type)
-      && VECTOR_TYPE_P (TREE_TYPE (ctx->ctor))
+      && VECTOR_TYPE_P (ctxtype)
       && index == NULL_TREE)
     /* A vector inside of a vector CONSTRUCTOR, e.g. when a larger
        vector is constructed from smaller vectors, doesn't get its own
@@ -6712,9 +6721,10 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx &new_ctx,
 	new_ctx.object = build_ctor_subob_ref (index, type, ctx->object);
     }
 
-  if (is_empty_class (type))
-    /* Leave ctor null for an empty subobject, they aren't represented in the
-       result of evaluation.  */
+  if (is_empty_class (type)
+      && TREE_CODE (ctxtype) != UNION_TYPE)
+    /* Leave ctor null for an empty subobject of a non-union class, they aren't
+       represented in the result of evaluation.  */
     new_ctx.ctor = NULL_TREE;
   else
     {

gcc/testsuite/g++.dg/cpp2a/constexpr-union10.C

@@ -0,0 +1,8 @@
+// PR c++/123346
+// { dg-do compile { target c++20 } }
+struct Unit {};
+union Union { Unit unit; };
+constexpr Union make(Union&& other) {
+    return Union {.unit = other.unit };
+}
+constexpr Union u = make(Union { .unit = Unit{} });