[PATCH v2] c++: further -Wdangling-reference refinement [PR107532]

[Marek Polacek <polacek@redhat.com>]

On Sat, Mar 18, 2023 at 08:35:36AM -0400, Jason Merrill wrote:
> On 3/17/23 16:29, Marek Polacek wrote:
> > Based on <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107532#c24>,
> > it seems like we should treat *any* class with a reference member
> > as a reference wrapper.  This simplifies the code so I'm happy to
> > make that change.
> > 
> > The patch, however, does not suppress the warning in
> > 
> >    int i = 42;
> >    auto const& v = std::get<0>(std::tuple<int&>(i));
> 
> Why not?  tuple<int&> has an int& member, doesn't it?  Do we need to look
> into bases as well?

Indeed.  I don't know why I didn't do it right away; it's really not that
complicated:

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Based on <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107532#c24>,
it seems like we should treat *any* class with a reference member
as a reference wrapper.  To suppress the warning in

  int i = 42;
  auto const& v = std::get<0>(std::tuple<int&>(i));

we have to look into base classes as well.  For std::tuple, this means
that we have to check the _Head_base subobject, which is a non-direct
base class of std::tuple.  So I've employed a DFS walk.

	PR c++/107532

gcc/cp/ChangeLog:

	* call.cc (class_has_reference_member_p): New.
	(class_has_reference_member_p_r): New.
	(reference_like_class_p): Don't look for a specific constructor.
	Use a DFS walk with class_has_reference_member_p_r.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference11.C: New test.
	* g++.dg/warn/Wdangling-reference12.C: New test.
---
 gcc/cp/call.cc                                | 63 +++++++++++--------
 .../g++.dg/warn/Wdangling-reference11.C       | 23 +++++++
 .../g++.dg/warn/Wdangling-reference12.C       | 12 ++++
 3 files changed, 72 insertions(+), 26 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference12.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index c52a09b9be2..429170e43ea 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13783,8 +13783,31 @@ std_pair_ref_ref_p (tree t)
 
 /* Return true if a class CTYPE is either std::reference_wrapper or
    std::ref_view, or a reference wrapper class.  We consider a class
-   a reference wrapper class if it has a reference member and a
-   constructor taking the same reference type.  */
+   a reference wrapper class if it has a reference member.  We no
+   longer check that it has a constructor taking the same reference type
+   since that approach still generated too many false positives.  */
+
+static bool
+class_has_reference_member_p (tree t)
+{
+  for (tree fields = TYPE_FIELDS (t);
+       fields;
+       fields = DECL_CHAIN (fields))
+    if (TREE_CODE (fields) == FIELD_DECL
+	&& !DECL_ARTIFICIAL (fields)
+	&& TYPE_REF_P (TREE_TYPE (fields)))
+      return true;
+  return false;
+}
+
+/* A wrapper for the above suitable as a callback for dfs_walk_once.  */
+
+static tree
+class_has_reference_member_p_r (tree binfo, void *)
+{
+  return (class_has_reference_member_p (BINFO_TYPE (binfo))
+	  ? integer_one_node : NULL_TREE);
+}
 
 static bool
 reference_like_class_p (tree ctype)
@@ -13800,31 +13823,19 @@ reference_like_class_p (tree ctype)
   if (decl_in_std_namespace_p (tdecl))
     {
       tree name = DECL_NAME (tdecl);
-      return (name
-	      && (id_equal (name, "reference_wrapper")
-		  || id_equal (name, "span")
-		  || id_equal (name, "ref_view")));
-    }
-  for (tree fields = TYPE_FIELDS (ctype);
-       fields;
-       fields = DECL_CHAIN (fields))
-    {
-      if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
-	continue;
-      tree type = TREE_TYPE (fields);
-      if (!TYPE_REF_P (type))
-	continue;
-      /* OK, the field is a reference member.  Do we have a constructor
-	 taking its type?  */
-      for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
-	{
-	  tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
-	  if (args
-	      && same_type_p (TREE_VALUE (args), type)
-	      && TREE_CHAIN (args) == void_list_node)
-	    return true;
-	}
+      if (name
+	  && (id_equal (name, "reference_wrapper")
+	      || id_equal (name, "span")
+	      || id_equal (name, "ref_view")))
+	return true;
     }
+
+  /* Some classes, such as std::tuple, have the reference member in its
+     (non-direct) base class.  */
+  if (dfs_walk_once (TYPE_BINFO (ctype), class_has_reference_member_p_r,
+		     nullptr, nullptr))
+    return true;
+
   return false;
 }
 
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
new file mode 100644
index 00000000000..667618e7196
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
@@ -0,0 +1,23 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct R
+{
+    int& r;
+    int& get() { return r; }
+    int&& rget() { return static_cast<int&&>(r); }
+};
+
+int main()
+{
+    int i = 42;
+    int& l = R{i}.get(); // { dg-bogus "dangling reference" }
+    int const& cl = R{i}.get(); // { dg-bogus "dangling reference" }
+    int&& r = R{i}.rget(); // { dg-bogus "dangling reference" }
+    int const&& cr = R{i}.rget(); // { dg-bogus "dangling reference" }
+    (void) l;
+    (void) r;
+    (void) cr;
+    (void) cl;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
new file mode 100644
index 00000000000..85e01f01a50
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference12.C
@@ -0,0 +1,12 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <tuple>
+
+int main()
+{
+  int i = 42;
+  auto const& v = std::get<0>(std::tuple<int&>(i)); // { dg-bogus "dangling reference" }
+  (void) v;
+}

base-commit: a226590fefb35ed66adf73d85cefe49048a78ab8
-- 
2.39.2