Re: [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]

[Marek Polacek <polacek@redhat.com>]

On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
> On 1/19/23 21:03, Marek Polacek wrote:
> > On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> > > On 1/18/23 20:13, Marek Polacek wrote:
> > > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > some grief.  The code in question uses a reference wrapper with a member
> > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > > 
> > > > > >      const Plane & meta = fm.planes().inner();
> > > > > > 
> > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > the case for member functions returning *this), but that then breaks
> > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > > 
> > > > > > So I figured that perhaps we want to look at the object we're invoking
> > > > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > > > warn about
> > > > > > 
> > > > > >      const Plane & meta = fm.planes().inner();
> > > > > > 
> > > > > > but do warn about
> > > > > > 
> > > > > >      const Plane & meta = FrameMetadata().planes().inner();
> > > > > > 
> > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > > 
> > > > > Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> > > > > reference-like type.  If planes() returned a class that held data, we would
> > > > > want to warn.
> > > > 
> > > > Sure, it's always some kind of tradeoff with warnings :/.
> > > > > In this case, we might recognize the reference-like class because it has a
> > > > > reference member and a constructor taking the same reference type.
> > > > 
> > > > That occurred to me too, but then I found out that std::reference_wrapper
> > > > actually uses T*, not T&, as you say.  But here's a patch to do that
> > > > (I hope).
> > > > > That wouldn't help with std::reference_wrapper or std::ref_view because they
> > > > > have pointer members instead of references, but perhaps loosening the check
> > > > > to include that case would make sense?
> > > > 
> > > > Sorry, I don't understand what you mean by loosening the check.  I could
> > > > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > > > what you meant.
> > > 
> > > Indeed that's not what I meant, but as I was saying in our meeting I think
> > > it's worth doing; the compiler has various tweaks to handle specific
> > > standard-library classes better.
> > Okay, done in the patch below.  Except that I'm not including a test for
> > std::ranges::ref_view because I don't really know how that works.
> > 
> > > > Surely I cannot _not_ warn for any class that contains a T*.
> > > 
> > > I was thinking if a constructor takes a T& and the class has a T* that would
> > > be close enough, though this also wouldn't handle the standard library
> > > classes so the benefit is questionable.
> > > 
> > > > Here's the patch so that we have some actual code to discuss...  Thanks.
> > > > 
> > > > -- >8 --
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief.  The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > > 
> > > >     const Plane & meta = fm.planes().inner();
> > > > 
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > 
> > > > Perhaps we want to look at the member function's enclosing class
> > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > member and a constructor taking the same reference type) and don't
> > > > warn if so, supposing that the member function returns a reference
> > > > to a non-temporary object.
> > > > 
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > 
> > > > 	PR c++/107532
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* call.cc (do_warn_dangling_reference): Don't warn when the
> > > > 	member function comes from a reference wrapper class.
> > > 
> > > Let's factor the new code out into e.g. reference_like_class_p
> > 
> > Done.  Thanks,
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > -- >8 --
> > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > some grief.  The code in question uses a reference wrapper with a member
> > function returning a reference to a subobject of a non-temporary object:
> > 
> >    const Plane & meta = fm.planes().inner();
> > 
> > I've tried a few approaches, e.g., checking that the member function's
> > return type is the same as the type of the enclosing class (which is
> > the case for member functions returning *this), but that then breaks
> > Wdangling-reference4.C with std::optional<std::string>.
> > 
> > Perhaps we want to look at the member function's enclosing class
> > to see if it's a reference wrapper class (meaning, has a reference
> > member and a constructor taking the same reference type, or is
> > std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> > supposing that the member function returns a reference to a non-temporary
> > object.
> > 
> > It's ugly, but better than asking users to add #pragmas into their code.
> > 
> > 	PR c++/107532
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* call.cc (reference_like_class_p): New.
> > 	(do_warn_dangling_reference): Don't warn when the member function comes
> > 	from a reference_like_class_p.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/warn/Wdangling-reference8.C: New test.
> > 	* g++.dg/warn/Wdangling-reference9.C: New test.
> > ---
> >   gcc/cp/call.cc                                | 48 ++++++++++++
> >   .../g++.dg/warn/Wdangling-reference8.C        | 77 +++++++++++++++++++
> >   .../g++.dg/warn/Wdangling-reference9.C        | 21 +++++
> >   3 files changed, 146 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> >   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> > 
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index 991730713e6..672722998ee 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> >     return true;
> >   }
> > +/* 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.  */
> > +
> > +static bool
> > +reference_like_class_p (tree ctype)
> > +{
> > +  tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> > +  if (decl_in_std_namespace_p (tdecl))
> > +    {
> > +      tree name = DECL_NAME (tdecl);
> > +      return (name
> > +	      && (id_equal (name, "reference_wrapper")
> > +		  || 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;
> > +	}
> > +    }
> > +  return false;
> > +}
> > +
> >   /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> >      that initializes the LHS (and at least one of its arguments represents
> >      a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> > @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> >   	if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> >   	  return NULL_TREE;
> > +	/* An attempt to reduce the number of -Wdangling-reference
> > +	   false positives concerning reference wrappers (c++/107532).
> > +	   Here we suppose that a member function of such a reference
> > +	   wrapper class returns a reference to a non-temporary object.  */
> > +	if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > +	    && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> > +	    && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
> 
> Ah, in this case I was thinking rather than return we would want to look
> through to the initializer of the reference wrapper, and warn if that's a
> temporary, so we can catch the *2 cases in your tests.
> 
> So, treating ref-like classes as much like references as we can.  Some of
> your v1 patch ought to be useful in implementing this, but only looking
> through one call at a time, not all of them like that patch.

Maybe this one, then?  I still have to loop through the calls though; EXPR in
do_warn_dangling_reference can be e.g.

Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)

or

Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)

and we want to warn only about the latter, but that means that I need to
look into the nested call 'planes' to see if the initializer was a temporary.

With this, we warn for the *2 cases too.

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

-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief.  The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:

  const Plane & meta = fm.planes().inner();

I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.

Perhaps we want to look at the member function's enclosing class
to see if it's a reference wrapper class (meaning, has a reference
member and a constructor taking the same reference type, or is
std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
supposing that the member function returns a reference to a non-temporary
object.

It's ugly, but better than asking users to add #pragmas into their code.

	PR c++/107532

gcc/cp/ChangeLog:

	* call.cc (reference_like_class_p): New.
	(do_warn_dangling_reference): Don't warn when the member function comes
	from a reference_like_class_p.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference8.C: New test.
	* g++.dg/warn/Wdangling-reference9.C: New test.
---
 gcc/cp/call.cc                                | 70 ++++++++++++++++-
 .../g++.dg/warn/Wdangling-reference8.C        | 77 +++++++++++++++++++
 .../g++.dg/warn/Wdangling-reference9.C        | 21 +++++
 3 files changed, 167 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 5715a7cd1de..137870670e7 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
   return true;
 }
 
+/* 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.  */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+  tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+  if (decl_in_std_namespace_p (tdecl))
+    {
+      tree name = DECL_NAME (tdecl);
+      return (name
+	      && (id_equal (name, "reference_wrapper")
+		  || 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;
+	}
+    }
+  return false;
+}
+
 /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
    that initializes the LHS (and at least one of its arguments represents
    a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13850,7 +13889,36 @@ do_warn_dangling_reference (tree expr)
 	    if (TREE_CODE (arg) == ADDR_EXPR)
 	      arg = TREE_OPERAND (arg, 0);
 	    if (expr_represents_temporary_p (arg))
-	      return expr;
+	      {
+		/* An attempt to reduce the number of -Wdangling-reference
+		   false positives concerning reference wrappers (c++/107532).
+		   Here we suppose that a member function of such a reference
+		   wrapper class returns a reference to a non-temporary object.  */
+		if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+		    && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+		    && i == 0
+		    && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
+		  {
+		    /* Let's see what the class object was initialized with.
+		       ARG is the TARGET_EXPR initializer; it may look like
+			 TARGET_EXPR <D.2839, A::foo (decl)>
+		       or
+			 TARGET_EXPR <D.2839, A::foo (&TARGET_EXPR <...>)>
+		       We should only warn for the second case.  */
+		    while (handled_component_p (arg))
+		      arg = TREE_OPERAND (arg, 0);
+		    arg = TARGET_EXPR_INITIAL (arg);
+		    /* Walk the call chain to the original object and see if
+		       it was a temporary.  */
+		    while (TREE_CODE (arg) == CALL_EXPR)
+		      arg = tree_strip_nop_conversions (CALL_EXPR_ARG (arg, 0));
+		    if (TREE_CODE (arg) == ADDR_EXPR)
+		      arg = TREE_OPERAND (arg, 0);
+		    if (!expr_represents_temporary_p (arg))
+		      break;
+		  }
+		return expr;
+	      }
 	  /* Don't warn about member function like:
 	      std::any a(...);
 	      S& s = a.emplace<S>({0}, 0);
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+    const T& i_;
+    Ref(const T & i) : i_(i) {}
+    const T & inner();
+};
+
+struct FrameMetadata {
+    Ref<const Plane> planes() const { return p_; }
+
+    Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+    const Plane & meta = fm.planes().inner();
+    bar(meta);
+    const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+    bar(meta2);
+}
+
+struct S {
+  const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+  D d;
+  Ref<const D> get() const { return d; }
+};
+
+struct B {
+  C c;
+  const C& get() const { return c; }
+  B();
+};
+
+struct A {
+  B b;
+  const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+  const auto& d1 = a.get().get().get().inner();
+  (void) d1;
+  const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+  (void) d2;
+  const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+  (void) d3;
+  const auto& d4 = a.b.get().get().inner();
+  (void) d4;
+  const auto& d5 = a.b.c.get().inner();
+  (void) d5;
+  const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+  (void) d6;
+  Plane p;
+  Ref<Plane> r(p);
+  const auto& d7 = r.inner();
+  (void) d7;
+  const auto& d8 = Ref<Plane>(p).inner();
+  (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+  std::reference_wrapper<const X> wrapit() const { return x; }
+  X x;
+};
+
+void
+g (const S& s)
+{
+  const auto& a1 = s.wrapit().get();
+  (void) a1;
+  const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+  (void) a2;
+}

base-commit: 327d45c57ebd2655a7599df0f01b8b5e2f82eda7
-- 
2.39.1