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

[Jason Merrill via Gcc-patches]

On 3/20/23 18:06, Marek Polacek wrote:

On Sat, Mar 18, 2023 at 08:35:36AM -0400, Jason Merrill wrote:

On 3/17/23 16:29, Marek Polacek wrote:

Based on ,
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(i));


Why not?  tuple 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?


OK.


-- >8 --
Based on ,
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(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 000..667618e7196
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
@@ -0,0 +1,23 @@
+// PR 

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

[Marek Polacek via Gcc-patches]

On Sat, Mar 18, 2023 at 08:35:36AM -0400, Jason Merrill wrote:
> On 3/17/23 16:29, Marek Polacek wrote:
> > Based on ,
> > 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(i));
> 
> Why not?  tuple 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 ,
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(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 000..667618e7196
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference11.C
@@ -0,0 +1,23 @@
+// PR c++/107532
+// { dg-do compile {