c++: constexpr and empty fields [PR97566]

In the discussion of PR98463, Jakub pointed out that in C++17 and up,
cxx_fold_indirect_ref_1 could use the field we build for an empty base.  I
tried implementing that, but it broke one of the tuple tests, so I did some
more digging.

To start with, I generalized the PR98463 patch to handle the case where we
do have a field, for an empty base or [[no_unique_address]] member.  This is
enough also for the no-field case because the member of the empty base must
itself be an empty field; if it weren't, the base would not be empty.

I looked for related PRs and found 97566, which was also fixed by the patch.
After some poking around to figure out why, I noticed that the testcase had
been breaking because E, though an empty class, has an ABI nvsize of one
byte, and we were giving the [[no_unique_address]] FIELD_DECL that
DECL_SIZE, whereas in build_base_field_1 empty base fields always get
DECL_SIZE zero, and various places were relying on that to recognize empty
fields.  So I adjusted both the size and the checking.  When I adjusted
check_bases I wondered if we were correctly handling bases with only empty
data members, but it appears we do.

I'm deferring the cxx_fold_indirect_ref_1 change until stage 1, as I don't
think it actually fixes anything.

gcc/cp/ChangeLog:

	PR c++/97566
	PR c++/98463
	* class.c (layout_class_type): An empty field gets size 0.
	(is_empty_field): New.
	(check_bases): Check it.
	* cp-tree.h (is_empty_field): Declare it.
	* constexpr.c (cxx_eval_store_expression): Check it.
	(cx_check_missing_mem_inits): Likewise.
	* init.c (perform_member_init): Likewise.
	* typeck2.c (process_init_constructor_record): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/97566
	* g++.dg/cpp2a/no_unique_address10.C: New test.
	* g++.dg/cpp2a/no_unique_address9.C: New test.

1 files changed, 25 insertions, 6 deletions

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 00c0dba..40f5fef 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -1835,15 +1835,13 @@ check_bases (tree t,
 	  else if (CLASSTYPE_REPEATED_BASE_P (t))
 	    CLASSTYPE_NON_STD_LAYOUT (t) = 1;
 	  else
-	    /* ...either has no non-static data members in the most-derived
-	       class and at most one base class with non-static data
-	       members, or has no base classes with non-static data
-	       members.  FIXME This was reworded in DR 1813.  */
+	    /* ...has all non-static data members and bit-fields in the class
+	       and its base classes first declared in the same class.  */
 	    for (basefield = TYPE_FIELDS (basetype); basefield;
 		 basefield = DECL_CHAIN (basefield))
 	      if (TREE_CODE (basefield) == FIELD_DECL
 		  && !(DECL_FIELD_IS_BASE (basefield)
-		       && integer_zerop (DECL_SIZE (basefield))))
+		       && is_empty_field (basefield)))
 		{
 		  if (field)
 		    CLASSTYPE_NON_STD_LAYOUT (t) = 1;
@@ -4226,6 +4224,25 @@ field_poverlapping_p (tree decl)
 			   DECL_ATTRIBUTES (decl));
 }
 
+/* Return true iff DECL is an empty field, either for an empty base or a
+   [[no_unique_address]] data member.  */
+
+bool
+is_empty_field (tree decl)
+{
+  if (TREE_CODE (decl) != FIELD_DECL)
+    return false;
+
+  bool r = (is_empty_class (TREE_TYPE (decl))
+	    && (DECL_FIELD_IS_BASE (decl)
+		|| field_poverlapping_p (decl)));
+
+  /* Empty fields should have size zero.  */
+  gcc_checking_assert (!r || integer_zerop (DECL_SIZE (decl)));
+
+  return r;
+}
+
 /* Record all of the empty subobjects of DECL_OR_BINFO.  */
 
 static void
@@ -6612,7 +6629,9 @@ layout_class_type (tree t, tree *virtuals_p)
 	  /* end_of_class doesn't always give dsize, but it does in the case of
 	     a class with virtual bases, which is when dsize > nvsize.  */
 	  tree dsize = end_of_class (type, /*vbases*/true);
-	  if (tree_int_cst_le (dsize, nvsize))
+	  if (CLASSTYPE_EMPTY_P (type))
+	    DECL_SIZE (field) = DECL_SIZE_UNIT (field) = size_zero_node;
+	  else if (tree_int_cst_le (dsize, nvsize))
 	    {
 	      DECL_SIZE_UNIT (field) = nvsize;
 	      DECL_SIZE (field) = CLASSTYPE_SIZE (type);