c++: -Wdangling-reference and empty class [PR115361]

We can't have a dangling reference to an empty class unless it's
specifically to that class or one of its bases.  This was giving a
false positive on the _ExtractKey pattern in libstdc++ hashtable.h.

This also adjusts the order of arguments to reference_related_p, which
is relevant for empty classes (unlike scalars).

Several of the classes in the testsuite needed to gain data members to
continue to warn.

	PR c++/115361

gcc/cp/ChangeLog:

	* call.cc (do_warn_dangling_reference): Check is_empty_class.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-no-dangling6.C
	* g++.dg/ext/attr-no-dangling7.C
	* g++.dg/ext/attr-no-dangling8.C
	* g++.dg/ext/attr-no-dangling9.C
	* g++.dg/warn/Wdangling-reference1.C
	* g++.dg/warn/Wdangling-reference2.C
	* g++.dg/warn/Wdangling-reference3.C: Make classes non-empty.
	* g++.dg/warn/Wdangling-reference23.C: New test.

1 files changed, 7 insertions, 5 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 664088e..1ecf3aa 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14356,12 +14356,14 @@ do_warn_dangling_reference (tree expr, bool arg_p)
 	    if ((arg = do_warn_dangling_reference (arg, /*arg_p=*/true)))
 	      {
 		/* If we know the temporary could not bind to the return type,
-		   don't warn.  This is for scalars only because for classes
-		   we can't be sure we are not returning its sub-object.  */
-		if (SCALAR_TYPE_P (TREE_TYPE (arg))
+		   don't warn.  This is for scalars and empty classes only
+		   because for other classes we can't be sure we are not
+		   returning its sub-object.  */
+		if ((SCALAR_TYPE_P (TREE_TYPE (arg))
+		     || is_empty_class (TREE_TYPE (arg)))
 		    && TYPE_REF_P (rettype)
-		    && !reference_related_p (TREE_TYPE (arg),
-					     TREE_TYPE (rettype)))
+		    && !reference_related_p (TREE_TYPE (rettype),
+					     TREE_TYPE (arg)))
 		  continue;
 		return expr;
 	      }